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SVT-HD Tube Functions?

#21
JimmyM said:
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.
Click to expand...

Yeah, 1/2 of V2 is the Channel 2 preamp. The other half drives passive EQ into V5 and then to the power amp.

Also I think V3 and V4 are reversed in function. V3 does the bass/treble, and V4 does the mid semiparametric. Both are Channel 1 only. V4 also drives V5 and then to the power amp.

IOW, Channel 1 and Channel 2 are summed/added together at V5.
 
#22
okcrum said:
Yeah, 1/2 of V2 is the Channel 2 preamp. The other half drives passive EQ into V5 and then to the power amp.

Also I think V3 and V4 are reversed in function. V3 does the bass/treble, and V4 does the mid semiparametric. Both are Channel 1 only. V4 also drives V5 and then to the power amp.

IOW, Channel 1 and Channel 2 are summed/added together at V5.
Click to expand...
holy crap! that's confusing!
 
#23
JimmyM said:
holy crap! that's confusing!
Click to expand...

Lemme chart it, might be easier:

V1+V3+V4: Channel 1
V1: Channel 1 preamp + the bass and treble switched boost and cuts
V3: Channel 1 EQ (bass + treble pots)
V4: semiparametric mids

V2: Channel 2 preamp and EQ

V5: Cathode follower, drives power amp + external amp

The two input channels get combined at V5.
 
#26
In addition to the comments made, the cathode follower (6C4) is often a source of microphonic noise. Rubber rings that are installed around the tube to dampen vibrations are available. I haven't had a lot of success trying these. They helped but didn't solve the problem.

The 6C4 is half of a 12AU7, which is a very reliable substitute. To install a 12AU7, you would need a custom 7-pin to 9-pin adapter socket that would fit in the amp. I'm not aware of a commercial adapter that is currently available.

If you are replacing this tube, look for a 6C4WA also called 6100. This is ruggedized version of the 6C4 that is designed for service where high temperature, mechanical shock and vibration are encountered. It is not as prone to be microphonic as the 6C4.

There is also the 6C4W. It is better than the 6C4 but it is not as good as the 6C4WA.
 
#29
I'm still a little confused on what a lower gain tube (i.e 5751) would do in the phase inverter slot. I read both sides of the spectrum, but not sure which is right. Does a 5751 in the PI slot make the power tubes distort sooner or later at same master and input volume?

Thanks again.
 
#30
Groover said:
I'm still a little confused on what a lower gain tube (i.e 5751) would do in the phase inverter slot. I read both sides of the spectrum, but not sure which is right. Does a 5751 in the PI slot make the power tubes distort sooner or later at same master and input volume?
Click to expand...

Shouldn't have any effect unless the tube is *real* low gain. :) PI's run at a voltage gain of 1. As with the 6C4s, microphonics could be a problem, but unless the tube is outright bad, 5751 should make no difference. Running out of current in the tube should not be an issue with this circuit either.
 
#31
I'll be trying two types of 5751 in the PI as well as in V1 (or maybe V2). Anyway, a few different combos. The two types will be the GE 6751 5-Star, and the other will be a Raytheon Blackplate (windmill). I've read lots of good stuff on that last one especially, so I'm hoping for good results. Both those tubes are suposedly very low in noise and harmonics.

Per Dave's post earlier, it was my understanding that with lower gain in PI, the Power tubes break-up sooner, which made total sense, then reading some other forums, some were saying the higher gain PI would break up the power tubes sooner instead.

Hence the confusion. I'm ultimatily trying to get overdrive without having to blast too high in volume (db, not the knob... Lol)

I'll try obviously, and most likely other variables come into play, but I'm trying to understand the basic wave form behind this part of the amp (PI)
 
#32
Groover said:
Per Dave's post earlier, it was my understanding that with lower gain in PI, the Power tubes break-up sooner, which made total sense, then reading some other forums, some were saying the higher gain PI would break up the power tubes sooner instead.
Click to expand...

All this is confusing and as clear as mud :( .

I was referring to the slightly lower current capacity of the 5751, as compared to the 12AX7. When the amp is pushed, it is possible that the 5751 in the PI position will run out of current capacity and this will result in distortion. It probably won't make a difference but you'll never know till you try it.

Gain of the 5751 vs the 12AX7 will come into play more if you try it in the first stage.

In any event, phase inverters, in a real amp vs in theory, are never perfect. In most amps, changing the PI tubes does affect tone when they are pushed. The fun is in trying these substitutions in your amp and see what the results are.


David
 
#33
Yeah understand David. I was staying out of the tone difference issue and just sticking to the pure 'lectronic stuff.

I was saying that I understand the 1.8 dB(v or i) difference in the plate current capacity of the two types of tube, but don't believe it matters in this particular PI circuit.

There's actually an internal balance adjustment in this PI circuit as well so it's better than most.

Agreed on the higher gain stages showing more of a difference.
 
#34
Ah, hah! I think it clicked in my head on why the contradiction in regards to a lower gain tube in the PI vs a higher gain one and which distorts when, where and why...

My theory:

A higher gain tube (12ax7) in PI will cause the power section to distort sooner because perhaps the PI being higher gain, will distortion causing what follows (the power section) to recieve a distorted signal from the PI.

On the other side of the fence, a lower gain tube (5751) in the PI will cause the pre-amp tubes to distort because they will need to run higher to push the 5751. Once that happens, the distorted signal from the pre then gets amplified by the power section.

Again, this is my theory, which clarifies to me why the two opinions on the PI are both valid. It's just a matter of which combination of tubes used throughout the amp produce that harmony that you prefer, hence the need to try different tubes and different locations to see what works best
 
#35
The first 6C4WA tubes we used in the HD units were stinkin noisy... not microphonic only, but also just plain noisy. The SVT poweramp has a lot of gain, and a low input voltage, so noise is bad.

We had, IIRC an ongoing issue with that, which is one reason the 6C4 disappeared from all SLM SVTs.
 
#36
glad i didn't order one then ;)

honestly, since i have no idea what an svt sounds like without a microphonic 6c4, and i still love the sound of my blue line, i guess i shouldn't worry about it so much.
 
#37
Groover said:
Ah, hah! I think it clicked in my head on why the contradiction in regards to a lower gain tube in the PI vs a higher gain one and which distorts when, where and why...
Click to expand...

Groover, the gain in the PI (and other well-designed) circuit is really determined by the components external to the tube, not the tube itself. Otherwise, you'd never be able to make a bunch of them all with the same gain.

A tube without enough gain or the ability to make enough current to drive the circuit as it is designed will start to distort - I think that's beans' point about the different tubes.
 
#38
Ok, thanks again for further explanation. I popped the GE 5-Star 5751 in the PI last night as the "first tube to try" in that slot, it sounded very nice and old school to me.

This is my first tube amp, so I'll have to grow into learning how they behave and such. I did notice that it really did richen up once it warmed up for a half hour or so.

When I'd take the clank out the amp by adjusting the highs, etc, the amp sounded rich with a touch of fuzzy.

I don't know if that's because it needs Biased (likely) or if the 5751 is giving it that fuzz.

I'll definitely keep you guys posted, as it's a still a work in progress.
 
#39
I've done quite a bit of tube-rolling and here is this is the tube medley that makes my amp happy :D - This experiment helped me nail down that nice fat, vibrating and edgy tone with good overdrive that I've been trying to find for a while.

Pre-Amp Section Tubes:

  • A) V5 - 6C4 WA - Azram/Mazda/Mullar Box/Toast Plates
  • B) V4 - 12AX7 - RCA Long Black Plates, Square Getter
  • C) V3 - 5751 WA - Raytheon Black Plates, Windmill Getter
  • D) V2 - 12AX7A - Tung-Sol Reissue
  • E) V1 - 5751 WA - 5-Star GE Gray Plates 3-Mica

Power Amp Section Tubes:

  • F) V4,5,6,7,8,9 - 6550C - Sveltana Winged-C Sextet
  • G) V2, V3 - 12BH7A - RCA Long Gray Plates, Round Getter
  • H) V1 - 12AX7A/7025 - RCA Short Gray Plate, Round Getter

The 5-Star GE 5751 in the V1 slot to really add some weight to the sound. Good solid bass - nice and meaty.

The Raytheon 5751 in V3 Midrange spot is clean and smooth. With that in there the mids appear to me to have a bit of a surround sound type of sound and sends out some good vibes

The RCA Short plate 12AX7A/7025 in the phase inverter slot ("H" in pics) seemed to give the best overall performance. Good thump and response. I tried the Raytheon here and a Sylvania/RCA 7025 long plate here, but something was just missing with those in place.

All in all, these are all subtle changes in the big scheme of things. The other tubes mentioned certainly add some flavor to their tasks, but a lot of it is based on a what I think is logical based on each tube specs and tonal properties I've gathered from reading about them or their build.

The Tung-Sol reissues are pretty nice too. Absolutely nothing wrong with them, and I'm sure if I would have retubed the preamp with all Tung-Sols off the shelf as I originally intended, the amp would still rock.

I do however feel the custom tubing assignment/layout does make a difference and does payoff. It just takes a little extra time and cash to gather some extra tubes to roll, but worth it if you like to do this type of thing.

And as others have said... if a tube doesn't sound just right in one spot, try it somewhere else. It may just be what that spot needs.

I hope that helps.


Image for reference:
attachment.jpg
 

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