SVT-HD Tube Functions?

[Groover]

This thread is to help me and others understand the tube functions of the mid 80's SVT-HD (Skunkworks)

Preamp Section Tubes:

• A) V5 - 6C4 - Follower Tube, the output stage of the preamp
• B) V4 - 12AX7 - Bass, Treble EQ channel 1
• C) V3 - 12AX7 - Semiparametric mid-EQ
• D) V2 - 12AX7 - Channel 2 EQ
• E) V1 - 12AX7 - Channel 1 Input

Power Amp Section Tubes:

• F) V4,5,6,7,8,9 - 6550 - Power Amp Tubes (Three Push-Pull Pairs: The two middle are a a pair, next two out are a pair, then two furthest apart "ends" are another pair)
  .
• G) V2, V3 - 12BH7 - Driver Tubes (Each tube drives 3 of the power amp Tubes, which means 1 tube controls the "Push" side, the other one drives the "Pull" side.)
  .
• H) V1 - 12AX7 - Phase Inverter Tube.

Disclaimer - The above references to the Vx (Valve Numbers) are based off a 70's schematic found online, and the other information has been added by use bases on discussion with other members on TB. I'll edit/add the information as additions/and or corrections are made.

Thanks for the help!

[Groover]

I'm particulary interested in the function of tube "C". When I got the amp, it was the only 12AX7 that was different than all the others in the amp... wondering if that one is "key" to the preamp tonal qualities, being that it's also shown offset (maybe for a reason?)

[Groover]

I take it this is a difficult question

[MrTAteMyBalls]

i guess so...lol. i have no idea, but if you haven't yet, shoot an e-mail to ampeg customer service or tech support. Fender USA has helped me track down schematics and parts and what not, even when i was not the original owner of the gear. they even sent me an input jack for a usa deluxe jazz that was out of warranty for free...so you never know.

[JimmyM]

if you go by the original design, tube c would be for the mid eq, i do believe.

[KramerBassFan]

Looking at a schematic would help you out here... more so than just having the tube layout...

[Groover]

Good deal. Thanks for the replies. I don't think there are schematics for the SVT-HD (Skunkworks) out there. Maybe, but I've never found one.

I'll contact Ampeg later today, but I think Jimmy might be on to something with the original layout (I think the amp should be a RI type deal, with some slight changes)

[Groover]

I found the schematic for the earlier SVT, and they appear to the same as the SVT-HD, except older ones use the 12DW7 whereas the 12AX7 are used in the HD for some slots - I think I can work with that.

I have a few tube choices I'm thining of using for the phase inverter slot (H)

The first choice is a 5751 5-Star command series GE. I found a NOS for dirt cheap (under $10!!) with perfectly matched triodes and readings of 1500/1500gm - These are supposed to be very rugged, and super quite operation - The gain on these is 70 vs the 100 on the 12AX7A, which is one reason why they are quite.

My other option would be a 1977 RCA NOS 7025 / 12AX7A with readings of 1400/1400 - these are also considered to be very quiet.

Being a phase inverter, I think the balanced triodes is more significant than higher gain and higher gm readings, right? If so, probably best to use the 5751?

Any opinions would be great.

I know, I know... try it and see what works best... lol

[Hoss]

E = channel 1 Input
D = channel 2 EQ = also 12AX7 in original 70s design
C = semiparametric mid-EQ
B = Bass, Treble EQ channel 1
A = Phase inverter

[Groover]

Hoss! How is it going?

So A is the phase inverter? I thought it was H... No?

[Hoss]

Of course H is the phase inverter.
I mixed it up - so A is the cathode follower, the output stage of the preamp!!!
sorry!

[beans-on-toast]

Originally Posted by Groover

I found the schematic for the earlier SVT, and they appear to the same as the SVT-HD, except older ones use the 12DW7 whereas the 12AX7 are used in the HD for some slots - I think I can work with that.

I would still take the time to contact Ampeg. I recall someone on TB mentioning that they got the schematics and a service manual from Ampeg. It would be nice to have the proper documentation for your amp, especially if you can get it for free!

I have a few tube choices I'm thining of using for the phase inverter slot (H)

The first choice is a 5751 5-Star command series GE. I found a NOS for dirt cheap (under $10!!) with perfectly matched triodes and readings of 1500/1500gm - These are supposed to be very rugged, and super quite operation - The gain on these is 70 vs the 100 on the 12AX7A, which is one reason why they are quite.

My other option would be a 1977 RCA NOS 7025 / 12AX7A with readings of 1400/1400 - these are also considered to be very quiet.

Being a phase inverter, I think the balanced triodes is more significant than higher gain and higher gm readings, right? If so, probably best to use the 5751?

Perfectly balanced triodes are not really needed in the SVT. It doesn't hurt but you don't need to pay for a balanced tube. There is an adjustment pot (VR3) that is used during the calibration process to balance the phase inverter triodes.

Any noise at the input, including that generated by the tube itself, is amplified at each subsequent stage. That is why low noise tubes like the 7025 are typically used in the first stage as the input tube. It wouldn't hurt to use this tube as a PI but it isn't worth the extra cost considering that the noise coming into the tube from the previous stages is going to be a lot higher than the noise generated by the 7025.

For $10 bucks, the 5751 is worth checking out. One of the important things to consider when selecting a phase inverter tube is the current output capacity. Often tube types with a higher current capacity are chosen for the PI. When they are pushed, as can happen in the output stage, and the current capacity runs out, the output stage distorts. The 5751 is close in spec to the 12AX7. Less gain but also slightly less current output capacity, 1 mA for the 5751 vs 1.2 mA 12AX7. In theory, when you push the PI 5751, your output stage will distort slightly sooner. The dynamics of the amp will change. You would have to try the two tube types in your amp and see what really happens. If you want a dirtier sound, the 5751 might help you out. Otherwise, you can try it in the pre-amp.

[Groover]

Great info. Thank you. This is really all starting to make sense. It's an ongoing learning process, and honestly, I'm becoming a tube junkie

So would it be best to probably try the 7025 (or the 5751 in the pre-amp slot labeled E?) Being that that is I think the first tube? Maybe not?

Sorry, my amp is still in parts. I usually post from work anyway, so it's all in my head...

[beans-on-toast]

Originally Posted by Groover

So would it be best to probably try the 7025 (or the 5751 in the pre-amp slot labeled E?) Being that that is I think the first tube? Maybe not?

Sure I'd give it a shot. They will both give you a cleaner sounding pre-amp. The 7025 in terms of hiss, the 5751 in terms of distortion. And try them in the PI position as well. It is fun to try different tubes and see what effect they have. Sometimes you make an interesting discovery. The extent of any noticeable changes will depend on your amp though.

One thing that you have to keep in mind when it comes to swapping tube types, don't rule anything out based on trying just one tube. Within any type, there are good and bad tubes out there. Sometimes when you get a good deal, and sometimes even when you pay a lot for a tube, you could be buying a noisy or microphonic tube. That is why finding a good honest dealer is important.

[Groover]

Originally Posted by beans-on-toast

.......One of the important things to consider when selecting a phase inverter tube is the current output capacity. Often tube types with a higher current capacity are chosen for the PI. When they are pushed, as can happen in the output stage, and the current capacity runs out, the output stage distorts. The 5751 is close in spec to the 12AX7. Less gain but also slightly less current output capacity, 1 mA for the 5751 vs 1.2 mA 12AX7. In theory, when you push the PI 5751, your output stage will distort slightly sooner. The dynamics of the amp will change. You would have to try the two tube types in your amp and see what really happens. If you want a dirtier sound, the 5751 might help you out. Otherwise, you can try it in the pre-amp.

Is there a certain type of 7025/12AX7/12AX7A/5751 that has higher current capacity? Do long plates, short plates, different brands come into play, or is the current capacity a fixed value depending on the tube (like 1.2ma for a 12AX7 no matter what)?

Does mutual conductance matter when it comes to the current? Say you have a tube that was "tested" at 42/42 and another tested at 48/48 on a Hickok tester (where minimun is 32/32 for this tube), would the 48/48 provide more current capacity, or are those numbers just a sign of the life of the tube?

Also, does anyone know of a cross-compare-table for various tube tester results? Like some testers will give a value like 1600gm, whereas another tube tester will give 44. How can you compare?

[beans-on-toast]

Originally Posted by Groover

Is there a certain type of 7025/12AX7/12AX7A/5751 that has higher current capacity? Do long plates, short plates, different brands come into play, or is the current capacity a fixed value depending on the tube (like 1.2ma for a 12AX7 no matter what)?

The simple answer is that you can have tubes from different companies all labeled 12AX7 and the specs can be different. You have to consult the spec sheets for each brand. Also within each brand, the specs of the same model will vary. Even within manufacturing runs. This is why some sellers sort and rate their tubes.

There is a 12A?7 family of tubes. Each have different specs and relatives. Some of them are: 12AU7 (mu=17-20), 12AV7 (37-41), 12AY7 (42-44), 5965A (45-47), 12AT7 (60-70), 5751 (70), 12AZ7 (59-60), 12AX7 (100-115). The differences are not just amplification factor. Some are more microphonic or noisy than others. There are different plate resistances, etc. Manufacturers have produced many versions of the 12AX7. In addition to the 7025 there is the 12DT7, 12DM7, 12DF7, 6681, 7729, and 12AD7. These are all low hum/low noise versions of the 12AX7. Versions were developed for specific applications such as military, mobile communications, space, and home appliances. Variations were developed to be cycled on and off, long service life, temperature extremes, power efficiency, high altitude, etc. The list goes on. All their spec vary to some extent.

People often substitute the 12AT7 for the 12AX7 in phase inverter applications. The 12AT7 has a plate current spec of 10 mA, much higher than the 12AX7's 1.2 mA. I doubt that you would gain anything by substituting an 12AT7 for the 12AX7 phase inverter in your SVT. As I said earlier, if the 12AX7 PI is running out of current capacity, substituting with a tube that can provide a little more plate current might provide you with a little more headroom.

In the case of your SVT, power amp stage is composed of the 12AX7 phase inverter, the 12BH7 drivers, and then the power tubes. The 12BH7 tubes have a relatively high plate current capacity (11.5 mA), even more so than the 12AT7. This allows them to fully drive the power tubes.

There is a problem. Modern tubes tend to not meet the spec of the NOS tubes. So if you buy a modern 12AX7, although the spec says it should have a mu of 100, there is a chance that your new tube is going to have a lower mu. Let's say 85 as an example. As a result, your amp is going to be performing under spec and could be distorting at lower volumes. If your 12BH7 tubes are under spec and can't produce the specified plate current, again, lack of headroom and distortion.

Does mutual conductance matter when it comes to the current? Say you have a tube that was "tested" at 42/42 and another tested at 48/48 on a Hickok tester (where minimun is 32/32 for this tube), would the 48/48 provide more current capacity, or are those numbers just a sign of the life of the tube?

This is getting complicated but in an attempt to answer your question... Mutual conductance is not based on a single parameter like measuring a voltage. Plate resistance and amplification factor tie into it. Mutual conductance = amplification factor/plate resistance. It is tying together the voltage of the input grid, the plate current and the plate voltage. These are all things that effect the conduction of the tube.

The bottom line is that a 48/48 is not necessarily better than a 42/42. The plate resistance may not be same and this will affect how the tube works. If the 48/48 tube had a low plate resistance than spec, the gain would be lower. You have to test them in your amp to know which you like best. All this complicates things.

Also, does anyone know of a cross-compare-table for various tube tester results? Like some testers will give a value like 1600gm, whereas another tube tester will give 44. How can you compare?

When you see a number like 44 you are looking at an amplification factor (mu). A number like 1600 is mutual conductance (gm).

The value of a tube tester is in comparing one tube to another. Even then it can let you down because they often do not test the tube under the same conditions such as plate voltage or load that it sees in the amp. Using a tester is not as good as testing the tube in the amp itself.

Testers do not adhere to a standard. If you read the same tube on multiple testers you would get results that would not match. This makes it difficult to compare results. There are some good testers out there like this one (http://www.amplitrex.com/).

[Groover]

Thanks again Beans. I really appreciate the time you've take to lay it all out here. I think I have a decent understanding now of what things mean and how to interpret them at least

I plan on doing a bit of tube rolling once things are back up and running and report my findings here.

I know ultimately doesn't matter what is where as long as it gets the tone in our heads, but this tube art is certainly intriguing to me.

Thanks again.

[Groover]

I made the edits to the original post with what members have contributed and what seems to be correct so far.

For the sake of my understanding and hopefully help others in the future, I have a few more questions in regards to the pre-amp tube section:

So far we have this:

A) V5 - 6C4 - Follower Tube, the output stage of the preamp
B) V4 - 12AX7 - Bass, Treble EQ channel 1
C) V3 - 12AX7 - Semiparametric mid-EQ
D) V2 - 12AX7 - Channel 2 EQ
E) V1 - 12AX7 - Channel 1 Input

I'm looking at the schematics, and honestly, I get about 50% of it, so the other half is really left to my interpretation - That said, I need some clarifications...

I'll start using the "Vx" numbers as that seems to be the better way to match this to the schematics.

So here is my understanding, so please correct me if I'm not interpreting it correctly:

V1 (E) is simply for channel 1. The tube with the character you like best should go here, as this "sets the tone" for all tubes to follow.

V2 (D) This being channel two, perhaps place a tube with opposite properties of what you have in V1. So say you have a nice warm smooth tone in channel 1 because of the type of tube used there, then put a high gain aggressive sounding tube in V2 so you have a "clean channel 1" and a "dirty channel 2, or at lease emphasize it by taking advantage of tube types.

V3 (C) - This controls the tone of the Mids knobs as well as how it interacts with the 220, 800, 3000 Hz selector switch in Channel 1. This tube does not come into play when using Channel 2, correct?

V4 (B) - Once again, this just operates off of Channel 1, and trying tube types here only affects bass and treble knobs for channel 1

Not sure if the above is correct or not, but if you are reading this and know differently, please post your interpretation.

[JimmyM]

i think you might be mistaken. channel 2 has to have a preamp tube. makes no sense to have a tube driven eq but no preamp tube.

[Groover]

^ and yes, I see what you are saying...