Peavey Heritage VTX

lets go work on a guitar amp,

this is a tube amp on the pwr section, 6L6, and chips and dips, chains and whips for the preamp,

how do you join the two?

rip off musicman, challenge your engineer to use power transistors in the tube cathode circuit,

always a pleasure to work on a Peavey, logical layout and designed for service, usally only 4 or 5 connectors, not 10,000 like Egnater and Fender DeVille,

we had some blown transistors in the cathode circuit so we replaced all four,

web bloggers say that you must use p/n 6465=MJ4247 or it won't bias right, we find this not to be true, we used

MJ15015 and 2n5550 instead of MPS6530 (2N3642)  and it is working great with more modern higher voltage parts,

this circuit has since faded from use as it does not sound that great and can be unreliablke when a tube blows,

shown are the two pwr transitors and their voltage clamps (small transistors)

oh wow, didn't notice the UL output,

weird using one screen resistor on one tube and not the other,

lots of diodes, eh?

they could probably use some 15 volt regs in the pwr supply for dependability,

Frankenstein called and want's his amp back....  :

Are there any initials for engr in the schematic label block? I might know the guy who did that...  Sondermeyer was the senior engineer, but he typically had a regular engineer to the day to day grunt work..

JR

here is the print block,

there was a blog about how the engineer did this circuit just to prove that he could do it, or something to that effect,

CJ said:

here is the print block,

there was a blog about how the engineer did this circuit just to prove that he could do it, or something to that effect,

Click to expand...

OK the drawn by and cb check is probably Ronnie Goss... Back in the early '80s he was Jack Sondermeyer's top (probably only) mechanical designer. Ronnie was the 3rd co-inventor, along with Jack and me on the heat sink patent years later.   

The final Approved by is the man himself (Jack Sondermeyer RIP). I don't recognize the check by... There was a Jack Aubuchon working there back then but he was a mixer guy not a tube amp guy.. Probably a junior analog engineer who left before I got to know Jack's people. Back in the early '80s I was working at a free standing division (AMR) 25 miles away, so we didn't hang out much.

[edit] oops brain fart... in the early 1980s I was still working up in Hartford CT,  I didn't end up at Peavey until years later 1985. Those 3 initials are pretty much who I said, and I definitely didn't know the third guy.  He was years before my time. [/edit]

That amp design looks like one of Hartley's "what would happen if you did this" ideas.  And Sondermeyer would say of course we can do that. Not that we should.... just that we can.  Jack was an old tube guy before he was the transistor guy at RCA.


JR 

> That amp design looks like one of Hartley's "what would happen if you did this" ideas.

A super-similar plan is in 2nd(?)-generation Music Man amps. Tube-BJT cascode with opamps pulling the reins. I believe the MM pre-dates this schematic.  Chassis 2100-130 may be 1974-79.

> weird using one screen resistor on one tube and not the other

The idea of screen resistors to protect the screens seems to come later. May come from TV-sweep experience, and didn't leak-out to the audio crowd at the time.

It is well-known that directly parallel tubes can fall into push-pull oscillation on parasitic reactances. It typically does not take many ohms to damp this down. Many more-serious amps run 50r plate resistors too. However just one may work, especially when not really in the output path.

Do V1 and V4 connect to the transformer with longer wires?

PRR said:

> That amp design looks like one of Hartley's "what would happen if you did this" ideas.

A super-similar plan is in 2nd(?)-generation Music Man amps. Tube-BJT cascode with opamps pulling the reins. I believe the MM pre-dates this schematic.  Chassis 2100-130 may be 1974-79.

Click to expand...

OK looking at those schematics without my beer goggles on the similarities are inescapable. Ignoring the glass bottle for output stage, that low voltage op amp front end driving grounded emitter high voltage level shifter intermediate stage was so widely used by Peavey for low cost solid state amps (like inside powered mixers and install amps) that in the early '90s I was tasked with investigating reducing the low voltage front end to a custom integrated circuit.  That same topology was also used by Crown in a series of install amps. Commonly known as "driven rail" for the variant I speak of, the low voltage front end drives the tops of the PS rail caps and speaker draws power from the transformer center tap that is swinging at the output audio signal voltage.

The later evolved amplifier version I was working on (to integrate) only used a single op amp front end-driver, with a OTA wrapped around it for DDT (clip limiting). The OTA was what tripped up my junior IC design engineer, he literally didn't understand how OTAs work, and I had to explain why simply adding emitter degeneration resistors to a long tail pair, wouldn't replace delivering a tightly matched LTP. That project went downhill after that, and real IC development costs lots of money, if more than just a trivial low voltage op amp with a few resistors. 

I recall back in the '70's seeing op amp front ends combined with level shifters to make higher voltage amps. I even built a DIY a hifi for my little sister using one of the several variants (built into the chassis of an old blown power amp to re-use the controls and power supply). Old school power amps used discrete front ends to handle higher voltage, and some still do, but the op amp front end was already not unique in the early '70s when I used it. I probably saw it in a design idea printed in one of the electronics trade magazines   

https://pdf.yt/d/51PM6w1n7kJHzRJC Here is a link to an amp design article co-authored by Jack Sondermeyer back in 1966 showing some of the subject topologies, only lacking the op amp front end, because in the mid 60's op amps were more like expensive science fair projects. I recall 709s(?) from the late '60s that cost upwards of $100 and would release their smoke if you looked at them side-ways. In my judgement and experience Jack didn't need to borrow amp designs from anybody, but Hartley was not above such (probably why he recruited Jack from RCA in the late '60s). 

I thought Wayne had a whole thread about op amp level shifters but I couldn't find it just now.

JR

> looking at those schematics without my beer goggles on the similarities are inescapable

More copycat than coincidence. PV had an MM open on the bench.

Background: Leo Fender was sitting-out a no-compete clause with CBS. MM started up, and may have released a similar amp with very traditional tube driver. Clause expired and Leo was suddenly elected to lead MM. The opamp+BJT driver appeared somewhere in there. I do not know if Leo had breadboarded it while "on vacation" or if it was developed inside MM. For other reasons, the original MM was falling apart by the time the PV seems to have been introduced.

It does give a new way to drive the output tubes into AB2. At the time, tubes were considered more reliable than transistors (Sondermeyer's insight into transistor protection was not yet widely known) and field- (tavern-) replaceable. But my take was that it might combine the worst parts of tubes and transistors. The voltage stress on the BJT is not clearly limited in overdrive. Total supply voltage must be larger to include drive voltage. Real gain of power through AB2 requires heroic voltages on tubes and OT. Complexity, and BJT and OT costs, suggest an all-transistor output would be a better commercial product.

The PV design echoes the MM design.

Opamps are inverting with "+" inputs at ground.
Opamps AC-coupled to transistors.
Transistors are diode-biased.
Overall NFB is inverting.

I had penciled this design without knowing the priors. There are other ways to skin this cat. The opamps could just as well manage the bias current, perhaps more accurately. Non-inverting mode would be a more traditional NFB loop.

The PV design differs in many details. Sondermeyer's influence shows in current protection for the BJTs (but no VA limiting, which he knew well). The bias is 1 diode higher and appears to be less-likely to go wrong. There's details which are not clear to me; as CJ sez, lots of diodes. Oh, and OT is ultralinear. It isn't a *copy*, but the "research" is clear, as is some deep thought and after-guessing.

I had one of these for a long while - purchased new and not at all cheap back then, in Australia.

Damn thing was forever blowing a bunch of big ass diodes and burning up stuff. After a couple of times at the repair shop, tres costly, and several new sets of tubes, I offloaded it.

It was my first tube amp, at the time (very early eighties), and I  knew virtually nothing about anything - as one does when they are in high school 

Last time I bought anything from that particular manufacturer. I sold it cause it was unreliable, back breaking and I heard of another amp which, at the time,  I thought sounded awesome - a Twin Reverb.

My memory now is that the Heritage sounded very dull.  I did like the phaser however, with it's 'stationary' setting - I used it alot. Probably to mask the sucky basic  tones this amp limped out with!

The speakers were quite good though, it had those Black Widow units.

The Twin Reverb, I remember was a lot brighter, which I liked more by the time I left high school.  Now of course, with my old-time ears,  I find the 70s Twins crazy offensive bright. My current and Goldilocks push-pull  amp is the Super Reverb.

JohnRoberts said:

OK the drawn by and cb check is probably Ronnie Goss... Back in the early '80s he was Jack Sondermeyer's top (probably only) mechanical designer. Ronnie was the 3rd co-inventor, along with Jack and me on the heat sink patent years later.   

The final Approved by is the man himself (Jack Sondermeyer RIP). I don't recognize the check by... There was a Jack Aubuchon working there back then but he was a mixer guy not a tube amp guy.. Probably a junior analog engineer who left before I got to know Jack's people. Back in the early '80s I was working at a free standing division (AMR) 25 miles away, so we didn't hang out much.

[edit] oops brain fart... in the early 1980s I was still working up in Hartford CT,  I didn't end up at Peavey until years later 1985. Those 3 initials are pretty much who I said, and I definitely didn't know the third guy.  He was years before my time. [/edit]

That amp design looks like one of Hartley's "what would happen if you did this" ideas.  And Sondermeyer would say of course we can do that. Not that we should.... just that we can.  Jack was an old tube guy before he was the transistor guy at RCA.


JR

Click to expand...

OK yesterday I got an email out of the blue from Jack Aubuchon the JLA on the signature block... Before he joined my newly formed Mixer engineering group at Peavey as my senior engineer he worked for Jack Sondermeyer in Peavey's Analog engineering group (that did mixers and powered mixers before my group was spun out).

He shares some insights about the VTX that put it in perspective.

Jack sez said:

"Drawn By:" and "C.B. Check:" is signed by REG, Ronnie Goss. "Check By:" is signed by JLA, Jack Aubuchon. "Appr. By:" is signed by JCS, Jack Sondermeyer.

The Main PCB was laid out by Jack Sondermeyer. He is the only one who would release a PCB without a Datecode and Initials. Hartley had not laid out a PCB since the the Bandit (warship series). The tube board is signed by KBB, Ken Bullock(?). Ken was a junior engineer in the department for a while. I don't know who the initials PJJ stand for. We would occasionally put initials on boards in recognition of help provided by production folks.

The glaring unanswered question was "Why use this power amp topology?" By this time, all of the reputable vacuum tube manufacturers had shut down operations. Peavey was forced to buy garbage vacuum tubes from from wherever he could find them. It became necessary to sort through troves of parts in order to find ones that would function properly. He literally had thousands of tubes in the warehouse that, for one reason or the other, were unusable. Hartly did not like to throw stuff away.  One day he commented to Sondermeyer that it would sure be nice if there were some way to use up all of those "bad" 6L6s. Sondermeyer came up with the cathode drive design as an option. It was a well known technique for driving power tubes. It wasn't widely used in a musical instrument amplification because it linearized the tube operation and destroyed all of the tonal qualities that the vacuum tube guitar amplifiers were known for. The diodes in the power transistor drive circuit, in addition to biasing the transistor devices, were added to simulate the effect of tube compression. If you read the schematic notes, it mentions defeating the simulated compression effect to allow 5% distortion measurements.

One additional tid bit. The circuit functioned well regardless of the operating characteristics of the individual 6L6s. Sondermeyer demonstrated on several occasions that the circuit would function just as well if the 6L6s were replaced by a jumper between the plate and the cathode. There was even some talk for a while of selling a "special" 6L6 "replacement" just for the VTX which consisted of a socket and a jumper. It was never marketed for obvious reasons.

Jack Aubuchon

Click to expand...

[edit]- in case it isn't obvious DO NOT jumper between plate and cathode... [/edit]

Jack sez said:

All of the "lifetime buys" and "speculative investment" caches of "quality" 6L6s had dried up. The market vacuum spawned  several startup companies that would buy loads of the inferior quality tubes. They  would select "good ones" and resell them - usually in matched pairs, with a huge markup. ( "Marginal" matched pairs work better than a marginal tube paired with a "good" one.)  Hartley felt that he could select "good" 6L6s just as well as those guys - cheaper.

I also recall that someone tried to reopen one of the old 6L6 factories because the parts were so scarce (Sylvania I think) but, by then, the recipe had been lost.

Hence, the VTX.

Eventually, military grade parts from behind the iron curtain became available and the VTX was no longer necessary.

Click to expand...

So apparently this made sense in that context at that time.... (early 1980s)

Jack Aubuchon is a brilliant analog engineer, and Jack Sondermeyer was a serious tube guy, before becoming a serious transistor guy.  I used to lecture a class at Peavey dealer seminars I called the "Jack and Johnny show", where me and jack Sondermeyer would explain new Peavey technology. I used to joke during his introduction that Jack forgot more about circuit design than I ever knew,  : teasing him about his age, but I was only half joking. He was an engineer's engineer... RIP

JR

Fascinating!

There was even some talk for a while of selling a "special" 6L6 "replacement" just for the VTX which consisted of a socket and a jumper.

Click to expand...

Hey they could have sold it to the audiophool guys. "Straight wire gain" 8)


Just looked at that music man schematic...700V on the plates

This topology would work fine jumpered P-K _if_ the transistor would stand the stress.

But it was picked for 1/10th the voltage and 1/10th the dissipation, so it won't.

The voltages the MM ran at, I don't think *any* transistor available then would live. If 1979 happened again, I guess I would look at TV H-sweep devices, but hope is slim.

Old tube guys and young tube guys do not know just how far down tubes went for a few years. I gave-away and even dumpstered a lot of heavy stuff because I figured it would not be maintainable. I can well believe PV had to hoard a warehouse of glass to get a few products out the door.

yeah some good stuff up above, thanks for that! 

you can replace the cathode drive circuit with a 12AT7, it is a bit of work but once done, should prove to  be more trouble free, and there are a lot of nice sounding amps out there that use vacuum tube phase inverters,