High Gain - Cold Bias

[trem]

Wish i would have looked at the schem, but i know the Train Wreck Express had a high value cat resistor on one of its triodes...right before the PI maybe.?
This Peavey 6505+ Head is very noisy on The Lead Channel. I spent quite a bit of time comparing pre tubes to make sure that tubes was not the issue.
I do not play guitar, but i have heard A Lot of amps. To me, the noise sounds like "High Gain Hiss". Not 60 or 120 hum, not a bad connection, not poor lead-dress... just high gain hiss that gets worse as you turn up the Pre or Post knobs.
Looking at the lead channel, a few of the pre tubes use (if i can see the numbers correctly) some pretty big value Cat Resistors. Both Channels share an 82k but then the Lead Channels sees preamp tubes with cat resistors of.....82,39 and 82K.
Assuming i am right and these tubes are biased cold, why is that in "High Gain" amps.?
BTW.....why do those resistors have a P1 after their values.?
Thank You

http://el34world.com/charts/Schemati...vey/5150-2.pdf

[eschertron]

I looked at the parts list (page 3?) because the cap values also looked wonky. What's printed on the schem is some kind of code. R17, for example, is 1.82k 1/4W 1% by the parts list, not 82k as we both thought looking at the schem. Thanks for posting this. First time I've seen this kind of strange.

edit: I see now a 1 then space then 82k. Zooming in there's no decimal point that I can see.

[Chuck H]

Well, one of them sure looks to say 82k, and another is certainly 39k but the last in that series looks more like B2k--- to me. Anyway, I'll bet it's a typo and both the coded resistors are 2k-ish. Have you measured them? The 39k is the same cold clipping stage used by Soldano and others. Many amps were fashioned after the SLO circuit it seems. It's a very high gain amp and I would expect the hiss. The 5150 is known for it in fact (from what I've read). I don't know that the cold stage contributes extra hiss. I don't see how it would, but I sure don't know everything.

[trem]

Sorry to make a long story out of this. My little, calculator sized, Audio Gen pooped out on me. I buttoned this back up and told the guy to bring it back after my new Audio Gen gets here. He had a broken coupling cap and a Loop Jack that was Not sticking open all the way, but not fully closing either......Intermittently.
I fixed those two things, but the Lead Channel sounds atrocious to me. Not sure if you guys have ever Played/Heard one of these...maybe that is "normal".?
I was not exactly saying the bias was adding to the hiss, just that i have seen the Cold Bias and High Gain marriage before.
The plate resistors are only 1/4 watt i believe.
Yeah, i thought C4 was maybe 50nF but it is 1MFD according to the parts list. The schem that Peavey sent me is actually a lot harder to read than this Hoffman Link.
It also says 1.82k for those cat resistors. My fault, sorry,
.....never seen a schem "coded" like this before.
Let me ask this, with a 39k cat resistor, and no bypass cap, the bias would be fairly high at idle...so a fairly large negative bias voltage on the grid.?
The tube might/probably cuts off at some point. or at least exhibits some pretty big clipping.?
What is the purpose of these big cat resistors.?
Thank You

[Chuck H]

The cold clipper can do a couple of things. In the TW circuit it actually limits voltage swing on the top end of the gain adjustment. On the SLO it's just part of the clipping voicing. Clipping one half of the waveform and then re-clipping it makes for a very smooth tone with a good balance of 2nd order harmonics (I've never put this to the test on bench gear though). I've done some experimenting with it myself and these are my personal, by ear observations. It's just a voicing thing. Gain stage structuring not withstanding. Most of high gain circuits are more about signal processing anyway.

[trem]

Ah.....OK.
Thank You Sir

[PeanutNore]

A tech at Peavey sent me the PDF schematic for my 6505+ 112 combo, and I'm certain that there are no 82K cathode resistors, but there are 1.82K ones. The 39K one is correct, though. Like Chuck H said, it's biased cold to clip asymmetrically. It's something amps like the 5150 (6505) and Soldano SLO inherited from the JCM 800 2203/2204 design that they evolved from. In a 2203/2204, it's the second gain stage of 3, and it's a 10K resistor. In the 5150 and SLO, it's 39K because it's the 3rd stage (out of 6, in the 5150, or 4 in the SLO) instead of the second and there's a lot more gain in front of it.

The common thread in all of these amps is that their designs aren't really unique or novel, they're all evolutions of the 5F6A Bassman circuit that Marshall copied for the JTM45, and implement various techniques that started out as mods to get more gain and overdrive out of Marshall JTM45s and Super Leads - cascading the inputs into each other to get 3 stages instead of 2, adding a cold clipper stage for asymmetric clipping, turning the cathode follower into a gain stage, adding another 12AX7 tube in front of the whole thing. The JCM800 is on the conservative end - it's basically just a JTM45 with the input channels cascaded and the second triode biased cold, with a master volume stuck on the end. The 5150 lead channel is the most extreme of the 3 and uses all the tricks - the cathode follower is turned into another gain stage, theres a whole extra tube tacked onto it for 6 gain stages total, plus it uses a cold clipper. The SLO is somewhere in the middle, keeping the cathode follower of the JCM800 but sticking a single extra triode on the front end for 4 gain stages (before the tone stack, at least - 5 if you count the one after the effects loop).

[trem]

Great Info.....Thanks.
Have you Heard/Played a 6505.?
What accounts for all this Hiss/Noise on the Lead Channel.?
Is it simply all the gain stages, or is there a "problem" with this head.?
Thank You

[PeanutNore]

I own a 6505+, the 112 combo. Same preamp as all the others. A friend of mine has a block letter 5150 that I've played as well. The hiss is thermal noise from the many resistors early in the circuit being amplified by all the gain stages after them. Myself, plus most people I know who own and use 5150/6505 type amps never really take the gain knob past about 7 or so - there's not really any sane reason to do so, it just adds hiss without really getting any dirtier.

I would see it as a sign of a problem in a 6505 if the lead channel didn't hiss, frankly. I like the amp and I still use it, but lately I've been using the JCM800 type amps that I built more. There's not as much gain available, but there's plenty, and the entire range of the gain knob is actually usable, even if the spot I like best is just short of maximum.

[Pedro Vecino]

Cathode resistors in lead channel are 1K8 and 2K2 except 39K. Bypass capacitors, 1uF. The gain of the lead channel is usually used (metal players) at 5 and has an appreciable noise level. In this amp it is necessary to take care of the noise level of the preamp tubes to the maximum.

[trem]

Great...OK.
When i say Noise, i am talking at idle of course. When my friend plays it, the hiss is not anywhere near as annoying.
He installed 6 new EH AX7 just a few months ago. I have some new JJ (short plate i guess) that i subbed in, one at a time, very slowly, paying attention to hiss, and could tell no improvement.
I had the Pre on 6 and rotated the Post from 5 to 8. The Hiss was pretty astonishing. But again, i am a drummer and much more accustom to hearing.....
BF Fender
AC Vox
Plexi Marshall
.....type of amps.
So.....it looks like this guy has a properly functioning Peavey 6505+ Head.
Thanks Again

[Chuck H]

It wasn't uncommon for amps developed after the SLO and using similar gain structuring to include a simple diode noise gate in the circuit so that the hiss would be minimized unless the amp was passing a guitar signal. Some worked better than others but most were a little hinky. Noise gate type pedals started to become more popular also. A testament to the hiss that is to be expected

Hiss is indeed a by product of gain at this level and there's not much to be done about it. Keeping key resistance values lower and the addition of high pass/low pass filters tuned to mitigate hiss while still allowing the guitar to come through sounding more or less right as well as other things have been employed in high gain designs and it really hasn't helped a lot. Modern high gain amps hiss a lot and not much has changed about them since the SLO.

If your guitarist is bothered by it he might try a noise gate pedal. They work better than the built in circuits because the gate can be tuned to very tight parameters dependent on specific amp settings.

[trem]

Thanks Chuck.
I will show him your post. This entire thread actually.
Thanks Again

[d95err]

My understanding is that the cold biased stage is a way to produce a lot of clipping with very little actual gain (i.e. noise). By using a low gain/low headroom stage, we get as much clipping as with a regular stage, but with less noise.

Clipping can be achieved without any gain at all, by using clipping diodes, for example like in the Marshall 2205 and 2255 and many boutique high-gain amps like Cameron and Friedman.

The Peavey 6505+ should be terrible in terms of hiss, with effectively 5 gain stages in the preamp.

[Mike Sulzer]

My understanding is that the cold biased stage is a way to produce a lot of clipping with very little actual gain (i.e. noise). By using a low gain/low headroom stage, we get as much clipping as with a regular stage, but with less noise.

Clipping can be achieved without any gain at all, by using clipping diodes, for example like in the Marshall 2205 and 2255 and many boutique high-gain amps like Cameron and Friedman.

The Peavey 6505+ should be terrible in terms of hiss, with effectively 5 gain stages in the preamp.

Clipping to a low level such as with diodes does not improve signal to noise ratio. In property designed and set up sequence of amplifier stages, the SNR is determined by the signal and the first stage: the noise from the first stage and the signal are amplified together, and so the noise from the first stage is amplified more than the from any other stage. Of course, in most guitar amps, the series input resistor contributes more noise than the tube. The guitar itself also might be noisier the the tube.

Many tubes does not necessarily mean lots of noise; it depends on how you set up the gains. Turn al the gains controls, and of course it is noisy, and that noise is from near the input. There are other ways to adjust the controls that can be noisy as well.