Channel 2 on the 1969 was very similar to the B-15. The topology was the same, the tubes were different.
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Ampeg SVT 50th Anniversary Edtition
- Thread starter Clap Trap
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Agreed to the B15N similarity - but it was still same sounding (and designed) channel in 1975.
Unless we're talking about similarity to channel 2 on the B-15, I really don't hear SVT channel 2 as being B-15ish. I hear both of them as being designed for guitar players.
When I first heard vintage SVT I was amazed how the 2nd channel sounded "like my 1970s B15NF on steroids". It's quite close actually, with honky mid bump and that raw, unpolished attack. I can't tell it about the SVT-VR, which second channel sounds like first one just without mid control and backwards wired ultra lo switch, which acts like bass cut not bass boost actually.
The only differences I have caught is the power supply voltage for the preamp went up over time. Node H was wired into the power supply and the value of R49 changed. There are also differences in the way the pins were wired in the umbilical, but I don't think that changed the way the circuit behaved.
Was it covered in the schematics? I can't trace it.
Did changing the resistor change the sound of the CH2 or just compensated the higher voltage?
The preamp is driven off Node B of the power supply. According to the 69 schematic, Node B is 430V and R49 is 8.2K. The 6550 schematics I have show 462V at node B and R49 is 4.7K.
I would think the higher voltage would change the sound and feel of both channels.
My suspicion is the 50th AV is just a slightly modified SVT VR. But something to consider is the SVT VR preamp uses three 12AX7s and two 12AU7s to maintain the original gain structure, and the 50th AV uses five 12AX7s, so it could be a totally new preamp design...or they could have just used the MTI era mod that converted 12DW7 to 12AX7.
Here's what I mean by original gain structure. The vintage amps used three 12DW7 tubes in channel 1. 12DW7 has two dissimilar triodes; one triode has the characteristics of a 12AX7, and the other triode has the characteristics of a 12AU7.
The SVT VR goes in and out of 12AX7s and 12AU7s to recreate the gain structure of the three 12DW7s used in channel one of the vintage amps. As you might imagine this makes the signal path a bit more complicated.
The vintage amps also used a 6C4 tube to blend the two channels. A 6C4 has one triode with the characteristics of a 12AU7. The fourth 12AU7 triode in the SVT VR replaces the 6C4.
Channel 2 in both the vintage amps and the SVT VR is made up of a 12AX7.
The SVT VR leaves half of a 12AX7 unused.
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I actually prefer the basic tone and feel of channel two on my vintage amps, but unfortunately I find it is not flexible enough for my needs without a mid control; so I never use it.
Wasnex at what volume point does your ch1 start to distort ?
I can turn my '73 up to beyond 2pm and still it's clean, it's 'stupid' loud but still clean. I've always wondered that there was something not quite right with that.
If so it could be the R47 value, the power at node B or could it be the pot ?
I can turn my '73 up to beyond 2pm and still it's clean, it's 'stupid' loud but still clean. I've always wondered that there was something not quite right with that.
If so it could be the R47 value, the power at node B or could it be the pot ?
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Yeah, that's true. But also, in the old SVT you dont need to mod the two (of 4 total) 12DW7 places to put there 12AX7s. But you can swap 12DW7 for AX7 in 2 places without resistor change and any sound change - that would tell those two 12DW7s only use their higher gain half. The other two remaining DW7s need 3 resistor changes (R6, R7, R25) if you want to put there AX7 to operate to the same conditions as 12DW7s so they both probably use two halves.
I do also think that 50th AV would be a bit modded VR, that would be great ifd they'd copy MTI layout, but we haven't got 6C4 or 12AU7 there, so....? It would not stick 100% to the MTI gain topology.
It would be nice to have Q&A with the 50th SVT designer here to anwswer all these questions... Maybe Dino would help with it?
I have the same thing going on with my '74, but on the 2nd CH.
This gain level is dependent on the output level your bass puts out. I use a Yamaha TRB6P. It's an active bass but the output is not that hot.
My 69 hits max clean volume somewhere around 10:30-11:00. I need to turn the 87 up a little higher.
There could be a lot of reasons why you need to turn up past 2:00 to get any distortion. If the amp sounds like an SVT and has been stable, I would probably not worry about it.
All four of the 12DW7s in the vintage SVTs use both triodes. You are correct that two of the slots take 12AX7 without mods. The mod rebiases half of V1 and V3. V4 in the preamp and V1 in the power amp section do not require any changes, just plug in a 12AX7.
Thanks for the clarification. The question is, why they put 12DW7 there if AX7 was the no-mod direct replacement and more popular (even at that time)?
Yeah i took that into account as it's a standard passive P bass. The amp was re-capped about 5 years ago and i remember now that Beans sent me info on the PEC module. Now i have some time on my hands i'll look into changing that.
Pins 1,2,3 make up the low gain (12AU7) triode in a 12DW7. If you look at V1 in the power amp, note the low gain triode is the phase inverter (PI). Also note in cathode circuit there is a 15K pot used to balance the PI. By adjusting the PI balance, you are rebiasing the tube. I.E. you don't need to change a resistor here because there is a variable resistor already in the circuit.
Biasing small signal tubes is a bit over my head. My understanding is 12AU7 has lower voltage gain than 12AX7, but wants to pull more current. Note that increasing the cathode resistance in the low gain sections of V1 and V4 is probably necessary to keep the 12AX7 from running too hot, which would result in early distortion.
My guess is the low gain section of V4 in the preamp is biased so cold that it is not a problem to throw in a 12AX7. I am not certain that there is no absolutely difference in sound when changing tube types in this slot. I have rolled tubes in lot's of my amps, and I can often hear fairly noticeable difference between different types of 12AX7s.
Because Fender was using them and Ampeg wanted to be different. It was always a thing with them until the 80's, especially when Everett Hull owned the company. Of course, back in those days, all those oddball tubes they used were plentiful and easily found. That all ended when American and European manufacture ended, and it's still a problem occasionally today.
I have two late 70's heads and no matter which passive bass I've plugged into them, usually either a P bass or a Ric 4001/4003 I've also set the volume at about 10-11 o'clock, Gibson maybe even between 9 and 10. I always set my bass pickups fairly close to the strings so they're hot, even string choice can have an effect on volume, but two o'clock seems like your gain is down compared to most.
I swapped an early 70's head to 12AX7's once and the sound changed and not for the better, I also changed an early 80's lower gain Skunkworks head to 12DW7's thinking it would sound better because of what happened to the early 70's head and the sound was the same and the gain stayed the same, who knows? I used to set the Skunkworks head at about 12 o'clock for comparatively the same volume as the late 70's heads at 10-11. 70's SVT's are really high gain. I leave the 12DW7's in the heads. I use JJ's and have had no problems with them although I know a few people did with the early JJ's.
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