Peavey Hybrid

[Andy Le Blanc]

I was out to the barn for a safety meeting the other day when I rediscovered
an old beat to crap peavey TNT in the bone pile. Pitiful old heap been to half
the bars in the state banged around the back of the van, a few beers dumped
on and the knobs all busted and such, rode hard and put away wet.
So I get the thing out of the cab and into the shop to scrap and I find that
true to the peavey reputation the power side of the unit is still in great shape.
So much for garlic and stake thru the heart.
Its the old 160w @ 4 ohm version. I thought to my self that it would be a
hoot to try a tube pre, chassis space is no issue, and I could experience the
pure tonal bliss of 160w of pure peavey power

The power side is very easily input'd with pin and ground, and Ill not be using
the +/- 15 v supply to pre. What I dont know is the limitations of driving a
peavey power side. I was thinking of trying an ampeg B-15 pre, simple enough.
Two 12v tranny back to back to get a B+ and 6v/12v fil. v, easy to build,
easy to mount, and fab a PCB with the tube sockets mounted on it, inside.

Any suggestions? It looks like a great way to recycle pawn shop peavey's.

[M Fowler]

Crap Andy you know more about this stuff then most, so it is hard to recommend a preamp to you guy. But when I read you were thinking of the Ampeg B15 I agreed right away.

What were the early Peavey amps Lynrd Skyrd used? They were all tube amps and sounded great.

My favorite amps have been high powered bass amps with effect pedal in the front. Don't need anything else. I have been tossing the idea of a Rocket pre into a Fender twin power section for fun.

Mark

[Andy Le Blanc]

Peavey VT 200. A quad of 6550A, and a fairly unique tone section active feed back.
Just about the best, which is strange to say about peavey now.
Undying but not the best reputation for tone, fenders been off the mark for a while now too,
shareholders blow.

The pre isn't really the issue, how hard can you drive a solid state peavey power side?
What is too little drive? Its the assumptions that sink a project.
Peavey is actually very good at product support, what I think I need is a scheme
with a signal applied voltage conditions. I'll see what I can find.

[Andy Le Blanc]

Here's a snap, plenty O room, the PCB that held the controls and pre was
cracked from abuse but the inside is so clean,

[Andy Le Blanc]

A few more of the finished board and a test fit. It really is a B-15 pre.
The tone controls will be hand built, that module they used to have on the old
ampeg tone pcb is long gone unless someone has a stash. I've gone with a
form of diode bias. It makes the hook up strangely simple. The entire board
assembly is shop scrap, may be 5$ in resistors and caps. I'm planning on
6sl7 but have plenty O 6sn7 to try. Its down to bolt assembly and dressing
the signal path.

[Andy Le Blanc]

I've chosen to go with the knobs in a deep set wooden panel.

[M Fowler]

What no wood knobs The amp is getting more interesting per post.

Mark

[Andy Le Blanc]

No wood knobs yet, its a few tools away. I've had the same thought.
It's looking to be a low profile head when is it done. Projects like this are a
great chance to try ideas. It starts with a simple notion, then each problem
turns into chance to feature a solution, like the dash board. I agree that a
contrasting wood would be good, I'd have to turn them, not yet, thats a
whole other consideration.

[Andy Le Blanc]

Interesting day, two diode biased 6SL7 are not enough to effectively drive
the peavey power side. There's a reason why ampeg went with the inverter
that they did, a paraphase inverter makes a lot of gain. So, I'm still dedicated
to trying a diode biased gain stage as the pre, there is something in the presentation
that just sounds good. And, I'm also committed to use only two gain stages
per the B-15, it's the same as the second channel on a SVT, I've had too
many bass playing friends, that simple pre and tone set up is a concrete and
widely accepted bass voice. I've opted to solve the gain issue by using a
6SJ7 and converted the second stage to accept the tube. Then the gain was such that
the hum was unacceptable. I first thought that it was supply ripple, the plate
voltage is raised with a voltage double and I thought that it had to be that,
but raising the value of the PS filtering caps didn't make a significant change
so I scabbed together a dc supply and found that to be the solution.

Gain levels with the 6SJ7 are much more acceptable, (thank you Phil S)
and tone with this project so far is very nice, I was very skeptical at first
that this would be a success, it was hard telling not knowing, and it is a peavey.

[Andy Le Blanc]

Here's some notes to hand dress the tone stack used in a B-15, and a quik
note on how to dress a twin triode to use one half to provide a diode bias on
an octal socket. It might be useful in an Express, where you've got that
unused 12ax7 stage, it sounds real nice as the first stage in a super lead.

New issue popped up with the peavey, the ripple from the voltage doubler is
quit apparent, I'm going to revise the grounding, get the first power supply
ground point isolated from the rest of the circuit ground, get the signal input
off the chassis. The peavey power side is so quiet that you hear every poor
design choice you make, in a good way. Instant critique.

[Andy Le Blanc]

Revising the ground had a positive effect, the hum is greatly reduced, its
always nice to have dead silence but I think the back to back 12v tranny
power supply is just about the cheapest way to go and may be the root of
the issue, at least in this application. A plate supply that does not used a
voltage doubler may be beneficial, less ripple. I reverted to a cathode bias
to see if the diode bias, which is subject to heater voltage variation, was
contributing to the hum issue, there was no difference but I did notice that
when gain is increased by the use of a bypass cap the hum also increases.

[Andy Le Blanc]

Here's a rough of the circuit. Its nice and warm, pentodes aren't favored
because of their distortion figures, but in this case it makes for a very nice
bass tone. The diode bias looks weird but is very nice. There are some
issues with it, device characteristic vary and are never constant, changing
with voltage condition, but it sounds nice. This circuit can be done with any
number of pentodes, 6sj7, 6au6, ef86 etc... Great way to get a phat tone for
bass that's affordable. The TNT was free, left for dead, and all the parts I
used were left over, bone yard pickin's, or hand fabricated from scrounged
material, the component board is made from an old clipboard.
You will need to take care to provide separate grounding for the power rectifier and first
filter stage, and avoid using the chassis for the grid return of the input stage.

[Andy Le Blanc]

Ventilation.

[Andy Le Blanc]

In the chassis.

[Andy Le Blanc]

The hook up from the pre, very easy.