Slightly odd output stage configuration. Thoughts ?

Several amp and tone stack mods. Variable cathode bias, OD mod on CH2, OT replacement, speaker replacement.

blueguitar.org/new/articles/blue_gtr/amps/peavey/c30_origmod.pdf

--
Live Fast, Die Young and Leave a Pretty Corpse

"Arfa Daily"

** Peavey have been doing that since their first 4 tube ( 6L6GC) models.

See Peavey Deuce from 1975:

Strangely, the 6 tube models have 6 screen resistors.

** Same question has puzzled me for ages - I suspect it could be an error that has just got repeated over the decades. No-one but Peavey knows for sure. ** It better be a damn good switch, insulation and voltage rating wise - most makers who have such triode/pentode switching use a very hefty slide switch.

Also, the power output will of course drop in triode mode and each EL84 will dissipate more heat under overdrive. Combined with their use of a "concertina" phase splitter, I suspect that overdrive clipping will not be symmetrical.

Not recommended.

.... Phil

Yes Phil. I too wondered whether it was an error. At first glance, I thought that the two screen grids were commoned, and then taken back to a single resistor, but when I looked at the print layout diagram a bit more carefully, I could see that what appeared to be the case was that one screen went to the 'cold' side of the resistor, and the other went to the 'hot' side. A squint at the schematic then confirmed this ...

Yes indeed, my thoughts as well. Anode supply is quoted at 330v nominal, so a switch rated to 250v ac, as most are, should be good for that. At the end of the day, many tube amps use the same switch for the HT standby switching, as they do for the mains, and lots of them run with an HT of much more than

330v.

I hadn't considered that there might be any changes to the phase splitter output as a result of such a change. What leads you to think that this might be the case ?

Well, not something that I would normally recommend either, and the owner is happy to live without the mod. He just thought that he would like to give it a try if it was practical, whilst I was in there doing the other work. Apparently, he would like to use it in 'triode' mode when he is at home practicing. It's not actually a hard mod to carry out, and would be reasonably easy to implement with the current print layout. I think what I will probably do is to just 'hang' the mod in temporarily to see how it performs, before doing any chassis drilling for switch fitting. If it seems to work ok, then I'll finish it off. If not, I'll just recommend against it.

Thanks for your comments Phil. Useful

Arfa

Hi Meat. Yes, it is Blue Guitar mods that he wants to do. He is not interested in the changes to the tone stack. He wants the output bias changing from fixed grid to cathode auto, he wants the C4 cap value change in the overdrive channel, and he wants the input simplification mod, where all the C-R garbage between the I/P socket and the first 12AX7 grid is removed, and a simple series resistor and grid return resistor are put in their place, a la typical Fender and Marshall I/P schemes.

Arfa

"Arfa Daily"

** Nope.

Switches ( and relays) rated for AC operation are not suitable for DC voltages above about 24 to 30 volts at the same current. DC arcs are tough SOBs to break.

** All *sensible* designs switch the AC secondary of the transformer. Those that switch the DC rail directly often arc and burn - even though the current is generally only 100mA or so.

Fender ( for example ) did this in many older models, but chose a particular switch that had a very large opening clearance to survive the inevitable arcing - ESPECIALLY if the standby switch was ever operated under full power.

Some Boogie models made the massive blunder of having the standby switch parallel two electros ( one charged, one not) when it was closed. Splat.

** Please try reading what is there.

The PS dos not itself change ( ??) but the job it has to do does - cos the required grid drive to the EL84s is much higher in triode mode. Concertina ( single triode ) PS circuits have less output and clip asymmetrically compared to twin triode PS circuits.

** Wot a f****it.

It only drops the power by half or 3 dB.

.... Phil

"Arfa Daily"

** Here is my analysis:

The low value of 100 ohms in series with G2 has no significant effect on the operation of a 6L6GC.

Peavey designs have a large value resistor ( 300 or 400 ohms 10W ) in the feed to all G2s - unlike other amps that generally have a filter choke with low DC resistance.

So, the 100 ohm resistors are there for factory test purposes only - so staff can check if valves are drawing G2 currents within pre-determined limits. G2 current levels mimic cathode currents for a given valve type and brand.

By measuring the voltage drop across the 300 or 400 ohm resistor AND each

100 ohm in the G2 feeds - correct operation can be established.

Dunno about others - but I routinely check the voltage drop across each G2 resistor when servicing amps and replacing valves. It should be similar for all valves in an output stage.

.... Phil

"Arfa Daily"

** That will either lose you a lot of output power or cook that puny power tranny ( by going close to class A operation) if you don't do it the smart way.

Not recommended either.

..... Phil

On 2/25/2011 4:12 PM Phil Allison spake thus:

If you wouldn't mind indulging my relative ignorance for a minute, that "concertina" stage business got my curiosity up. So I found the two triode (12AX7) stages before the EL84s, and it looks to me like the first stage (V3A) is a phase inverter, with the other triode acting as

*another* phase inverter cascaded from the first stage, presumably with unity gain. Is that correct?

So it seems this setup would be quite problem-prone, with possible phase distortion and phase imbalance (or crossover distortion) effects. So why would anyone use such a phase inverter, when there are much better ones available?

--
The phrase "jump the shark" itself jumped the shark about a decade ago.

- Usenet

"David Nebenzahl" Phil Allison spake thus:

** With an imagination like that - you should be writing fiction for a living.

A " phase inverter" ( or phase splitter) has equal and opposite polarity outputs.

** Nope.

V3A is a voltage amplifier ( gain of about 50 before NFB) while V3B is the "concertina" phase inverter. Concertina circuits always have equal plate and cathode resistors and in operation, the voltage between the plate and cathode grows and falls with audio drive like a concertina being pumped in and out.

The cathode of V3A acts as a negative ( opposite phase) input for feedback from the output transformer secondary.

Because of the easy drive requirements of an EL84, such a concertina circuit is able to send them deep into overdrive - while maintaining good symmetry. Not so for 6L6GC or EL34 stages which need several times more drive voltage.

.... Phil

Well, that one is a well known and documented mod for that amp, so I'm not too concerned about doing it. However, I take (and somewhat agree with) your point. Personally, I would prefer to make the fixed grid bias variable within 'blues to rock' limits, and give him another knob to twiddle with, but he is adamant that he wants that particular cathode bias mod. I did offer to make that switchable for him, but he just wants it changing and leaving at that. You can only advise these people, but in the end, it's his amp, and if he's prepared to pay the money to have it done at his own risk, then that's fine by me ...

Arfa

So is it the fact that the circuit relies to some extent on NFB for correct symmetrical overdrive operation, that leads you to believe that this might be upset by operating in a pseudo-triode mode ?

Interesting analysis. I'll report back when I've tried it

Arfa

Looks like a good bit of work in store for you. The hard part is to end up with tone that doesn't sound like ass.

--
Live Fast, Die Young and Leave a Pretty Corpse

On 2/25/2011 11:29 PM Phil Allison spake thus:

OK, my bad: poor choice of terminology. What I meant to say is that both stages are inverting stages, being common cathode. Correct? Since one pair of EL84s seems to be driven from V3As plate and the other from V3Bs plate, isn't this basically how this phase splitter works? One pair of output tubes gets an inverted signal, the other pair gets an inverted-inverted one.

--
The phrase "jump the shark" itself jumped the shark about a decade ago.

- Usenet

"David Nebenzahl" Phil Allison spake thus:

** I already explained the circuit - but you decide to snip the lot.

Here is the schem:

Here is the missing explanation:

" V3A is a voltage amplifier ( gain of about 50 before NFB) while V3B is the "concertina" phase inverter. Concertina circuits always have equal plate and cathode resistors and in operation, the voltage between the plate and cathode grows and falls with audio drive like a concertina being pumped in and out. The cathode of V3A acts as a negative ( opposite phase) input for feedback from the output transformer secondary. "

** You looking at the schem above or not ?

V3A drives V3B which delivers TWO anti-phase signals (one from the plate and one from the cathode) so the push-pull output stage can work.

V3B has a gain of 2 as the output is across the plate and cathode.

.... Phil

I fixed the Subject line and edited your comment to reflect the truth.

No need to thank me.

--
Live Fast, Die Young and Leave a Pretty Corpse

Thank you, anyway.

How many times do we have to tell you, Phil, that we are neither impressed with nor frightened my your rude remarks?

Aren't you bothered that other people consider you psychotic? And to what end?

You haven't yet blocked him?

Jamie

I haven't any background experience to make suggestions, but this perplexing circuit you're seeing sounds a like the schematic I looked at for the

*Peavey Mace A* series (6, 6L6GC output tubes). That's sounds like how the schematic was drawn, although I haven't see the actual circuit layout of that model.. separate resistors were used for each tube of one pair.. then another pair was wired direct, while the third pair had 2 more separate resistors shown between those pairs (appearing like the 2 resistors were wired in series between those 2 pairs).

There seem to be numerous similarities in many PV amp model schematics of the power output sections, but then some specific oddities in a couple of models' schematics that look out of place (to me, anyway).

One thing that I have discovered recently, is that there are a huge number of PV amp users, and lots of forum discussions about many modifications (some that have produced results, and some that don't) and also repairs. You're probably already aware that many forum discussions wrt electronic circuits can include unintentional errors and/or other misleading info (this mod I came up with is the ultimate! for example), so that much of the forum info needs to be pondered and then either accepted or rejected by one's better judgement and/or experience.. and compared to the rule: if it werks, don't f*ck with it.

As I've only recently started dabbling in this area, I've noticed one thing that's prevalent.. practially every component in/on guitars and amps have been tampered with and replaced, and the majority of reports of the actors claim that the acts produce amazing results.

-- Cheers, WB .............

"Wild_Bill"

** That a good rule to live by....

** But they *would* say that - wouldn't they ?

.... Phil