peavey amp theory and design

[xjbear]

I would love to just have somebody that knows alot more than me to break down circuit by circuit how and why peaveys designs work the way they do. Im going to post a Butcher schematic (its about as stripped down as an amp can get) and would like to understand better how the power and bias section work. Why there is an fet in the power board and what it does all the way to the 100ohm screens 47k grid leaks and etc. Thanks for the help!


peavey-butcher.pdf

[xjbear]

Btw this is the OLD Butcher which shares the same bulk of design as the rockmaster, vtm, etc.

[Enzo]

If you want to discuss the Butcher, fine, but really, for a basic amp, look at the often built 5E3 Fender Deluxe. Doesn't get much simpler.


In the Butcher, there is no FET on the power board or anywhere in this amp. That is a plain old bipolar transistor. It turns on the LED CR8. That is the light associated with the standby switch. What controls that transistor? All it does is turn on the LED when high voltage is present in the tube circuits. In my own work, if I see that LED dark, I assume we have lost high voltage.

220k R45,46 connect the transistor base to the 500v rail, but there is also the 1k R54. Those three form a voltage divider: 220k+220k+1k, so at the base of the transistor we see 1k/(220k+220k+1k) x 500v, so 1/441x500 = 1.13v. So if 500v is present, then about a volt hits the transistor, turning it on.


None of that has anything to do with the screen resistors. The screens run off the 500v rail, and the LED looks at the rail, but they do not work together otherwise. If you snipped out the transistor, then the LED would not light. Period. Nothing else changes.


The only relationship between the transistor and the bias and grids is that the same power transformer winding happens to provide their operating voltage.

[tedmich]

I think Q1 transistor is just there to switch the "active" LED on when the amp comes off standby. It then drops some bias voltage through the LED. Merlin Blencowe's books are a great place to start learning about typical tube amp circuits:
http://www.valvewizard.co.uk/
see also Pete Millet's treasure trove of old tube books:
tubebooks.org - Vintage info from the age of vacuum tubes

[xjbear]

Well thanks a bunch enzo that hits all the points I needed hit. I have Merlins preamp design book and keuhnels power amp book already. I gotta good handle on all the fender and marshall circuits but for some reason this thing looked a little convoluted

[xjbear]

I really need to spend some time on power supplies. My migraine addled brain was thinking it might feed off b+ into the bias circuit to balance the bias or something weird but that still didnt make sense at all.
Ive got one in my junk pile that I need to dig out and fart around with. It blew a tube out of a new quad within 5 minutes of cranking hard on it, bias was good, threw another quad in it. That quad was showing 28ma at 515v. Blew out one of those too. Checked ot and one primary was 70 ohms the other was 40 ohms, in relation to ct. Put a new 2k marshall ot and hardwired the speaker jacks. Threw in a new set of tung sol 6L6gc (500plate/500screen ones) supposedly they can handle some abuse. Its idling at 36ma at 517v and im afraid to dime it and find out if its gonna blow anything else. Im having a feeling those ganged shunt jacks were the culprit and fried the ot somewhat.

I also saw this post where you mentioned dc elevated heaters in these amps. Maybe +25v. Is that to raise the plate to cathode potential or to just shave off some noise? And how do I check that its actually doing what its duppose to?

http://peavey.com/forum/viewtopic.ph...t=2991&start=0

[xjbear]

Also installed 1k screen resistors, removed flyback protection diodes. The previous time it blew a tube the plate voltage had arc'd to the heater trace so I dremeled it out and ditched the diodes. Pulled out the humdinger pot which now had burned up and replaced it with two 100ohm resistors to ground. Thats the history of if you have any golden advice just in case im missing something.
Speaker cable is perfect and running it into a new 1x15 4ohm 300watt cab.

[Enzo]

Why remove the protection diodes? Now you no longer have their protection for your output transformer.


By replacing the hum circuit with two 100 ohm resistors to ground, you have removed the DC offset for the preamp heaters. This may or may not increase hum. Did you not notice the wiper of the hum pot did not go to ground? Look at R55 in the power supply, it connects the heater AC to +26vDC for offset. If you want to use 100 ohm to ground virtual center tapping, then remove R55.

Do I think Peavey stuck that elevation in there just for kicks? No, not at all. DC elevation of heaters prevents heater to cathode currents, which would be 6v of AC and would become hum.


I suspect your original OT was not defective, uneven resistance is a common result of one winding method on center tapped windings.

[tedmich]

there is a meaningful difference between "fixing and amp" and "experimenting on a test mule" both are valid pursuits but I'd probably start with a simpler less powerful circuit than the Butcher, but if its what you have... I'd try tacking and hacking on the thru hole component leads and avoid overtaxing the thin and delicate PCB traces. The power supplies on Peavey amps have nothing extra IME, and modding there will gain you very little, its subtle changes in the audio path components which will give you the most insight into the art and science of tube amps. Be careful; first do no harm!

[xjbear]

The two 100ohm go to the 100uf cap then to ground so its effectively the same thing. Not wanting anything out of this amp except to throw fuzz on the front end and crank every knob to ten, reliably. I compared this ot's readings to the same part number in my vtm and the butcher one is way out. 34ohm difference betwe

[Dave H]

...I compared this ot's readings to the same part number in my vtm and the butcher one is way out. 34ohm difference betwe

34ohm isn't way out. The OT primary impedance will be several k ohm so it's small compared with that.

[xjbear]

Marshall dsl ot 19.1/19.2
Vtm 24.6/23.8
Weber jcm 800 replacement 21.2/21.5
Butcher tranny 31.2/71.3
???
And flyback spikes blasting out pcb between heater and plate. Id say that is a potential issue

[Enzo]

Yes, Dave is confusing impedance and resistance.

And my perfectly good Classic 30 OTs measure about 90 and 120 ohms, side to side. It all depends on how they wound for the center taqp, there are two ways, one results in differing resistances.



On the other hand, if we have some shorted turns, then very little spiking action can occur.

[xjbear]

Good to know! Does it make a difference which pair of pltes you wire it to, in respect to the grid signal from the pi? 82k side versus 100k screen load on the pi

[Dave H]

Yes, Dave is confusing impedance and resistance.

No he isn’t I was just trying to point out that the tubes are driving an impedance of several k ohms so it doesn’t really matter that the two halves of the OT primary differ by 34ohms which is as you say for your Classic 30.

There can be no fly back spikes because of a difference in resistance (you need inductance for that). If it doesn’t have negative feedback either power tube can be connected to the either the 82k or 100k PI output.