Ampeg 50th Heritage SVT 1969/1975 channels question

[boroman]

The question was asked before, just when the special edition appeared but it seems no one knew it back then.

It's advertised that 1srt channel is modeled after blueline SVTs (69-71) and second channel after 1975 models. Is this only a marketing thing? Because I don't see any differences in construction over the years for the preamp...?

 

[Wasnex]

The question was asked before, just when the special edition appeared but it seems no one knew it back then.

It's advertised that 1srt channel is modeled after blueline SVTs (69-71) and second channel after 1975 models. Is this only a marketing thing? Because I don't see any differences in construction over the years for the preamp...?

I think it's pretty much marketing BS, although I do believe the voicing of channel 2 is modified.

I think this post is still relevant: trying to understand differences in Ampeg 300W tube amps...

I haven't seen a schematic, so I still can't say for sure...Maybe @beans-on-toast can give a definitive answer.

 

[beans-on-toast]

Both Loud and Yamaha both used the original SlM circuit boards. They were redesigned for the Heritage models. After that, the regular models also used new boards. The ‘69 and mid 70’s based preamp design in the Heritage model is sonically different. This might help. The vintage amps had different tubes so changes were made to accommodate different tubes. In general, how do you voice a preamp? The plate voltages, component values and even the components. Channel 1 and channel 2 were always different topologies, the two circuit designs were different. The mid 70’s channel 2 had more headroom.

 

[beans-on-toast]

This is a very interesting interview with Bill Hughes.

 

[Wasnex]

Both Loud and Yamaha both used the original SlM circuit boards. They were redesigned for the Heritage models. After that, the regular models also used new boards. The ‘69 and mid 70’s based preamp design in the Heritage model is sonically different. This might help. The vintage amps had different tubes so changes were made to accommodate different tubes. In general, how do you voice a preamp? The plate voltages, component values and even the components. Channel 1 and channel 2 were always different topologies, the two circuit designs were different. The mid 70’s channel 2 had more headroom.

It's really a shame that they don't directly AB both channels in both amps with the same player and same bass.

I didn't really find the 75 channel to sound cleaner in the video. So I have to wonder if they mean more gain, rather than more headroom.

I have not played a mid 70s SVT, but many people seem think they had a bit more gain than the early amps. But I have seen conflicting comments that the amps running 6146s had more gain because the tube type requires more drive.

My experience: As I have mentioned I own a 69 and an 87 Skunk. The Skunk has the factory circuit changes to run 12AX7s in channel 1. This involves changing out 3 resistors and then swapping tubes, so its a simple and non-destructive update. The amps do sound a bit different, but I would say they both sound like SVTs. In order to match levels between both channels on the 69, the volume of channel 2 must be cranked way higher than channel 1. On the 87, the level differences between the two channels is not as extreme. I suspect channel 1 has a bit less gain than my 69 and channel 2 may have a bit more.

I believe the intent with the 75 channel on the 50th AV was to voice it like a 1975 channel 1. IMHO there is only so much you can do to achieve this since the topology of the two channels is drastically different. Channel 1 has 6 gains stages VS 2 gain stages in channel 2. Also channel 1 mixes tube sections with 100 gain factor and 20 gain factor, while channel 2 use 100 gain factor on both triodes. More importantly channel 1 has a mid control and channel 2 does not. In order to actually achieve the design goals they seem to imply, they would have needed to stuff two more tubes and a mid control into the available space. I think this would be difficult and cost prohibitive. (Or do I have this wrong, and it's intended to be a 1975 channel 2?)

I actually prefer the basic tone and feel of channel 2 over channel 1. But I never use it because, IMHO it needs a mid control for me to dial in a usable live sound. I have fiddled with running both channels. Although it's cool, I didn't find it all that useful for the type of sound I typically go for.

 

[Wasnex]

@beans-on-toast , have you actually seen the schematics for the 50th AV...or perhaps had a chance to poke around one?

 

[beans-on-toast]

@beans-on-toast , have you actually seen the schematics for the 50th AV...or perhaps had a chance to poke around one?

Looked inside, didn’t take readings or notes. Non-disclosure.

I will say that their description of the Heritage channel differences is vague and without specifics. They don’t want people changing voltages or components to build their own.

 

[ThisBass]

The Skunk has the factory circuit changes to run 12AX7s in channel 1. This involves changing out 3 resistors and then swapping tubes, so its a simple and non-destructive update.

The resistor in circuit V1 must not necessarily be changed. Most of all available 12AX7 do run probably in V1 position without any change of resistor for the buffer stage. Sonic wise there is not much of a difference to either 12AX7 vers 12DW7 cause 1/2 of the 12DW7 (one system) is already a 12AX7 type tube system.

The amps do sound a bit different, ...

Every tube amp sounds a little bit different once its tubes are swapped for a different tube.

I did notice some/little difference when I did go for 12AX7 in channel one instead of 12DW7 and I did the same way notice some/little difference when I did go for different 12Ax7 vers 12AX7 (kinda some tube rolling within its family).

Personally I think there is no need to draw "mystics" of sound off from very oldish SVT's history vers something that was then made later in the SVT's total longish history.

 

[ThisBass]

My experience: As I have mentioned I own a 69 and an 87 Skunk. The Skunk has the factory circuit changes to run 12AX7s in channel 1. This involves changing out 3 resistors and then swapping tubes, so its a simple and non-destructive update. The amps do sound a bit different, but I would say they both sound like SVTs. In order to match levels between both channels on the 69, the volume of channel 2 must be cranked way higher than channel 1. On the 87, the level differences between the two channels is not as extreme. I suspect channel 1 has a bit less gain than my 69 and channel 2 may have a bit more.

Channel one and channel two are totally different animals schematic wise that must not be compared each other by no means.

Besides buffer stages (channel one) the gain structure on both channels is totally different.
If any the tone stack for bass/treble EQ might be the same on both channels.

 

[Wasnex]

The resistor in circuit V1 must not necessarily be changed. Most of all available 12AX7 do run probably in V1 position without any change of resistor for the buffer stage. Sonic wise there is not much of a difference to either 12AX7 vers 12DW7 cause 1/2 of the 12DW7 (one system) is already a 12AX7 type tube system.


Every tube amp sounds a little bit different once its tubes are swapped for a different tube.

I did notice some/little difference when I did go for 12AX7 instead of 12DW7 and I did the same way notice some/little difference when I did get for different 12Ax7 vers 12AX7 (kinda some tube rolling within its family).

Personally I think there is no need to draw "mystics" of sound off from very oldish SVT's history vers something that was then made later in the SVT's total longish history.

The original SVTs have three 12DW7s in the preamp and one 12DW7 in the power amp. The factory mod to run 12AX7 appears to involve rebiasing two tube sections and adjusting signal attenuation at the output of V1. Nothing is done to the circuits for the third 12DW7 in the preamp or the 12DW7 in the power amp...you simply swap in 12AX7s.

I would not be surprised if the amp will run 12AX7s in all positions without changing the resistors...but my guess is it will perform closer to intended design if the update is done to spec. Otherwise I doubt MTI would deemed the circuit changes necessary.

To my ears, the 75 channel in the 50th AV still sounds like a channel 2, although I do think the voicing has been modified a bit in a way some may prefer.

I think including the 75 channel sort of pays lip service to what was actually requested... which I believe was to include two full-featured channels. I think including the 75 voicing was also done to set the 50th AV apart from the VR...from what I remember the VR was originally marketed as being based on the 69. So my impression is the 75 channel is mostly marketing BS.

I have rolled 12AX7 tube types in quite a few amps. Some circuits respond to tube rolling and some do not. Those circuits that do respond tend to respond in subtle ways. Some people don't think it is worth the effort, but I have found it to be very beneficial with some amps.

I only remember rolling 12DW7 in one amp (Ashdown CTM300. I rolled about five different brands including current product and NOS tubes. I didn't hear enough difference to justify the cost of a new tube...so I put the stock JJs back in. I suspect the tube type is not as varied as 12AX7...but it could be the circuits in the amp are not sensitive to tube rolling.

 

[ThisBass]

The original SVTs have three 12DW7s in the preamp and one 12DW7 in the power amp. The factory mod to run 12AX7 appears to involve rebiasing two tube sections and adjusting signal attenuation at the output of V1. Nothing is done to the circuits for the third 12DW7 in the preamp or the 12DW7 in the power amp...you simply swap in 12AX7s.

V1
The resistor for the output of the the buffer should be increased to run the buffer stage with 12AX7 type tubes.
Yet most of all available 12AX7 tubes do run properly without any change to the circuit cause these tubes still deliver (almost always) sufficient current to the task.

I would not be surprised if the amp will run 12AX7s in all positions without changing the resistors...but my guess is it will perform closer to intended design if the update is done to spec. Otherwise I doubt MTI would deemed the circuit changes necessary.

The gain at V2 stage goes noticeable high if a 12AX7 was placed without any schematic correction for V2. Also the THD at V2 may go high without any resistive adjustment to the schematic

To my ears, the 75 channel in the 50th AV still sounds like a channel 2, although I do think the voicing has been modified a bit in a way some may prefer.

They do sound "familiar" yet not the same.

I think including the 75 channel sort of pays lip service to what was actually requested... which I believe was to include two full-featured channels. I think including the 75 voicing was also done to set the 50th AV apart from the VR...from what I remember the VR was originally marketed as being based on the 69. So my impression is the 75 channel is mostly marketing BS.

Personally I try to take things just as realistic as possible, to me there is no need to overthink things too much, or just "a lil" bit beyond than there was need doing it.

I have rolled 12AX7 tube types in quite a few amps. Some circuits respond to tube rolling and some do not. Those circuits that do respond tend to respond in subtle ways. Some people don't think it is worth the effort, but I have found it to be very beneficial with some amps.

Some schematics respond more than others. That's the way how it is.
Ampeg amps normally don't respond too much to rollings, only a little, if any, yet they do.
And that's the reason why so many folks try to place not suitable tubes in ampeg amps where the schematic in question was not designed for.

 

[boroman]

Thank you all for the replies! I have never played Heritage model, so just asking. I owned few SVTs (1970, 1972, 1973, 1974, 1976, 1980 (MTI), 1982 (MTI) and three SVT-VRs.

I didn't see any notable differences in sound/gain between all those old stuff incl. MTI. I have 72 on the workbench right now, and yeah, you need to crank the 2nd channel way up to get same loudness comparing to first one.

There were minor changes to SVT during the production years (schematic says that too), but same sound was there. I did the AX7 mod to V1 (power amp splitter) and its no difference (as they say). I did the other DW7 swap for AX7 with the resistor mods - made no change to the sound for me. But if you direct swap it, it also works good, especially if you want your breakup a bit earlier. If the soundguy tells you at every show to turn it down, it might be the time to swap all the DW7s for AX7s.

SVT-VR 1st channel was very close to vintage SVT, but 2nd channel was way different in response, also in all three I had (one US made, two fat east), the ultra low switch on 2nd channel was wired backwards (anyone else noticed that?), also VRs distorted earlier.

 

[ThisBass]

Thank you all for the replies! I have never played Heritage model, so just asking. I owned few SVTs (1970, 1972, 1973, 1974, 1976, 1980 (MTI), 1982 (MTI) and three SVT-VRs.

I didn't see any notable differences in sound/gain between all those old stuff incl. MTI. I have 72 on the workbench right now, and yeah, you need to crank the 2nd channel way up to get same loudness comparing to first one.

There were minor changes to SVT during the production years (schematic says that too), but same sound was there. I did the AX7 mod to V1 (power amp splitter) and its no difference (as they say). I did the other DW7 swap for AX7 with the resistor mods - made no change to the sound for me. But if you direct swap it, it also works good, especially if you want your breakup a bit earlier. If the soundguy tells you at every show to turn it down, it might be the time to swap all the DW7s for AX7s.

SVT-VR 1st channel was very close to vintage SVT, but 2nd channel was way different in response, also in all three I had (one US made, two fat east), the ultra low switch on 2nd channel was wired backwards (anyone else noticed that?), also VRs distorted earlier.

Can't see much of a difference (vers "oldish" vintage ones) in the schematic of VR's second channel.
https://www.bustedgear.com/images/schematics/Ampeg_SVT-VR.pdf

VR schematic is also downloadable here:
Invalid Link Removed

The most obvious difference was done for the input impedance on both channels.
The vintage schematics do read 5.6Meg while the VR schematic reads 3.32Meg (both channels input impedance).
Although the difference reads big on paper, in real practice there will be no audible difference neither in level/gain nor in sound nothing at all.

If you do experience an very audible difference in sound with channel two of your VR, in first instance I'd guess that your VR was not completely up to specs, respectively there might be an issue with channel two of your VR.


The VR power amp schematic reads 0.4 Vrms input level for full output power (300 Watt rms).
That's very interesting cause the "oldish" SVT schematics do read 0.257 Vrms.

Long time ago there was a discussion on TB forum about the meaning of both diodes at the input stage of the power amp.
Many techs/experts did claim the diodes don't have much of an effect soundwise, cause the power amp does push its full power 300 Watts rms at "smallish" 0.257 Vrms (which corresponds to 0.364 Vpeak).

From what I can tell about my 77..78 SVT, the input level for full output power is noticeable higher than stated in the schematic (I still have got the original schematic that once was delivered with the SVT).
The input level is more close to ~4 Vrms rather than 0.257 Vrms, and I can tell that both diodes really do have an effect to the signal and sound when the amplifier is pushed hard to its full output power.

If any, then your VR should break up "a lil later" rather than "distort earlier".

 

[ThisBass]

Routing of signal path throughout the tubes at channel one.

12DW7 tube (and also 12AX7 tube) versions of the SVT


V1 - Input stage
- gain stage -> V1a - system 12AX7
- buffer stage -> V1b - system 12AU7/(12AX7)

V2 - Volume and tone stack (Bass/Treble)
- gain stage 1 -> V2b - system 12AU7/(12AX7)
- gain stage 2 -> V2a - system 12AX7

V3 - Midrange section
- gain stage -> V3a - system 12AX7
- buffer stage -> V3b - system 12AU7/(12AX7)


VR version of the SVT

V1/V2 - Input stage:
- gain stage -> V1a - system 12AX7
- buffer stage -> V2b - system 12AU7

V2/V3 - Volume and tone stack (Bass/Treble)
- gain stage 1 -> V2a - system 12AU7
- gain stage 2 -> V3b - system 12AX7

V3/V4 - Midrange section
- gain stage -> V3a- system 12AX7
- buffer stage -> V4b - system 12AU7


It can clearly be seen that any tube rolling (V1..V3/V4) that was done to the VR does have a completely different effect versus its older 12DW7 sister.
The signal path throughout "type of tubes" is remaining just the same with the VR

system 12AX7
system 12AU7

system 12AU7
system 12AX7

system 12AX7
system 12AU7

Yet the signal path throughout "tube systems" in particular along V1..V3 is different.

And I think it should be noticed that 12DW7 tubes may probably not be a good idea (not a good match at all) for channel one of the SVT-VR.

edit:
at least it should be possible to run the VR with 12AX7 tubes in V1..V3 position, provided the well known one (or two) resistor mods are done to channel one.
For V4 (with some chance) a 12DW7 might be a proper match, yet I didn't checked the pin layout

 

[ThisBass]

Personally I love my Ampeg stuff that I own.
I do love my ~78 SVT and I do love my SVP pro preamp

But, honestly, a 1969 voiced SVT channel #1 and a 1975 voiced channel #2, so how stupid and ignorant shall I be just to believe in that kinda vintage story full of crap?


At least to me it seems to be nothing but a marketing gag just to draw (new) attention to the holy grail of bass guitar amplification.
Even Behringer seems to be more "honest" in this regard!
Sorry to say, but that's what I think about it!

 

[beans-on-toast]

Personally I love my Ampeg stuff that I own.
I do love my ~78 SVT and I do love my SVP pro preamp

But, honestly, a 1969 voiced SVT channel #1 and a 1975 voiced channel #2, so how stupid and ignorant shall I be just to believe in that kinda vintage story full of crap?


At least to me it seems to be nothing but a marketing gag just to draw (new) attention to the holey grail of bass guitar amplification.
Even Behringer seems to be more "honest" in this regard!
Sorry to say, but that's what I think about it!

They did the same thing with the Heritage B-15. There’s a switch for preamps in two different eras, 1964 and 1966. The topologies are different and the channels sound different. What they did makes sense.

HB-15

In the case of the 50th anniversary SVT, the EQ is different. The bass and treble have the same centre frequencies and the same boost and cut. Channel 2 does sound different than the vintage channel 2. Personally, I like channel 2 on the vintage SVT, it sounds like a B15..

 

[ThisBass]

In the case of the 50th anniversary SVT, the EQ is different. The bass and treble have the same centre frequencies and the same boost and cut. Channel 2 does sound different than the vintage channel 2. Personally, I like channel 2 on the vintage SVT, it sounds like a B15..

Personally I think it makes no sense nothing at all to call it 1975 channel two.
My ~1978 SVT does have the same schematic on channel #2 just the same as it was for the early SVT that was introduced in 1969.
Besides some minor deviation for the input impedance the schematic remained just the same even for the VR version of the SVT.
The question that must be asked in this context, what the hell shall be different schematic wise for SVT's made in 1975?
If any, than 1975 made SVT's must be kinda sort of very rare birds soundwise on channel #2.

You are right, even me myself I do appreciate the sound of channel #2 cause it provides a more "airy" and open sound vers channel #1.

 

[Wasnex]

Personally I love my Ampeg stuff that I own.
I do love my ~78 SVT and I do love my SVP pro preamp

But, honestly, a 1969 voiced SVT channel #1 and a 1975 voiced channel #2, so how stupid and ignorant shall I be just to believe in that kinda vintage story full of crap?


At least to me it seems to be nothing but a marketing gag just to draw (new) attention to the holy grail of bass guitar amplification.
Even Behringer seems to be more "honest" in this regard!
Sorry to say, but that's what I think about it!

My impression is they sort of dropped the ball on the 50th AV. They were late getting it to market and apparently they carried over a design flaw from the VR that results in a bit too much hum in some amps...AFAIK, the standby switch is too close to preamp

I have seen way too many threads where the 50th AVs had problems including excess hum and loose screws rolling around in the chassis. I have seen no evidence of a factory fix for the hum problem, and my impression is they have not done a good job handling warranty claims on this model. AFAIK the solution is exchange rather than fix. While exchange would seem better in some circumstances, some TB members have received multiple bad amps and eventually gave up and cancelled the order.

While I think Ampeg overused marketing hype, I am more concerned with the apparent QC problems and substandard performance.

I don't really put this on Yamaha as I think development started before the sale, and Yamaha appears to have stayed out of it. However, the transition to the new owner almost surely complicated the development and release of the 50th AV.

I am happy with my 69 SVT, 87 SVT, SVT 8-Pro, and PF50T...but they each have noticeable flaws. I don't have much interest in a VR or or 50th AV, but I sometimes think it might be nice to have a CL.

 

[Wasnex]

Personally I think it makes no sense nothing at all to call it 1975 channel two.
My ~1978 SVT does have the same schematic on channel #2 just the same as it was for the early SVT that was introduced in 1969.
Besides some minor deviation for the input impedance the schematic remained just the same even for the VR version of the SVT.
The question that must be asked in this context, what the hell shall be different schematic wise for SVT's made in 1975?
If any, than 1975 made SVT's must be kinda sort of very rare birds soundwise on channel #2.

You are right, even me myself I do appreciate the sound of channel #2 cause it provides a more "airy" and open sound vers channel #1.

My guess is the difference in the actual 75 is the plate voltage was a bit higher. The 69 power amp schematic shows R49 at 8.2K. The mid 70s 6550 schematic I have show R49 is 4.7K.

The R49 feeds node B, which is the preamp supply.

 

[ThisBass]

My guess is the difference in the actual 75 is the plate voltage was a bit higher. The 69 power amp schematic shows R49 at 8.2K. The mid 70s 6550 schematic I have show R49 is 4.7K.

The R49 feeds node B, which is the preamp supply.

Not really
I do remember that I had to replace R49 (4.7k) on my my ~78 SVT long time ago (cause it was burned/fried).
Instead of 4.7k I did replace something ~8k for R49 without any noticeable difference soundwise to both channels #1 and #2, cause the preamp supply voltage didn't change too much.

edit,
IIRC I did in the long past a couple of AB-Testing about what does happen soundwise to the SVT preamp if the supply voltage does change.
The SVT preamp is a very robust preamp versus tube rollings at the one hand side but also as well pretty robust versus some voltage drift (voltage drop) on the supply voltage.