filthy dirty Peavey Classic VTX hybrid amp just handed to me, how do they sound?

[Dave H]

I've been studying the schematic but I still don't understand where the cathode gets this +65 volts that you mention. Help me out here if you don't mind

The voltage on the base of Q6 (and Q7) sets the voltage Vi (at about 30mV in the notes). Vi sets the cathode current to Vi/R126 = 30mV/5.6 = 5mA. The voltage on the cathode will rise to produce the necessary bias voltage to make the tube conduct 5mA of cathode current at the plate/screen voltage it has.

[Silvertone Jockey]

Got it. So it is something similar to putting a zener diode in series with the cathode to ground connection seen in many RF linear amps to develop bias (the only experiences I have to compare too)

Any recommendations as to where to obtain the needed parts? Peavey?

[Jazz P Bass]

Got it. So it is something similar to putting a zener diode in series with the cathode to ground connection seen in many RF linear amps to develop bias (the only experiences I have to compare too)

Any recommendations as to where to obtain the needed parts? Peavey?

I don't know about the zener analogy.
It's a cathode biased circuit.

As always, call Peavey.

[J M Fahey]

It is a cathode biased circuit, no doubt, but more precisely it's a common base amplifier (google it) , normally used only in RF transmitters and such.

The tube will pass exactly the current fed it by the cathode transistor, period.

Voltage cathode to grid will take whatever value is needed to ensure that.

Datasheets are usually thought as one way only, as in "I input grid(to cathode) and plate voltage and get tube current", but it works the exact same way if we input plate voltage and current and read grid (to cathode) voltage needed.
So in this case we apply 525V to plate, go along the 5mA line and we'll read the grid (to cathode) voltage needed.

Add to that the grid (to ground) voltage present, say for example 14V, and you'll find the actual cathode (to ground) voltage.

The transistor just feeds current and by itself can drop as low as 2V .

So this 6L6 can now have net from around -50V at idle, typical "Fender bias" voltages, to +12V , well into Class AB2 .

Now you know why this kind of amps pulls out so much power out of these tubes, and is quite insensitive to individual tube specs.

And uses them down to the last drop.

If a tube has any emission left, it will work here.

VERY clever design.

[Silvertone Jockey]

It is a cathode biased circuit, no doubt, but more precisely it's a common base amplifier (google it) , normally used only in RF transmitters and such.

So this 6L6 can now have net from around -50V at idle, typical "Fender bias" voltages, to +12V , well into Class AB2 .

Now you know why this kind of amps pulls out so much power out of these tubes, and is quite insensitive to individual tube specs.

And uses them down to the last drop.

If a tube has any emission left, it will work here.

VERY clever design.

I rest my case. All of the bold statements above describe exactly my experience with RF amplifiers, not that, that is the subject at hand.

An interesting side note is that it's common for some RF amps using the same 2 x 6L6GC tubes to achieve a hundred or so watts RMS, but the plate idling current is kept to extremely low levels with a variable bias circuit, which does cause some (usually ignored) distortion

[oc disorder]

Another question, does anyone have a link to any good semi conductor cross references? I did some searching but haven't come up with much other than manufacturer specific ones.

Here you will find the Peavey transistor crossreference

http://music-electronics-forum.com/t22020/

SJ-5884 = MJE-15030 TUBE DRIVER, REPLACE PAIR

MPS-6530 = 2N3642, MPS6531


I'm finding this site is useful for transistors

Bipolar Transistor Cross-reference Search | Equivalent Transistors

[Silvertone Jockey]

Here you will find the Peavey transistor crossreference

http://music-electronics-forum.com/t22020/

SJ-5884 = MJE-15030 TUBE DRIVER, REPLACE PAIR

MPS-6530 = 2N3642, MPS6531


I'm finding this site is useful for transistors

Bipolar Transistor Cross-reference Search | Equivalent Transistors

Thanks for the link, useful indeed

[Silvertone Jockey]

I love ebay. 10 x MJE 15030, fast and free 3 day shipping 13 bucks. 10 x 2N3904 $3 delivered 4 days. I haven't yet not found the part I'm looking for and often get the best deal without leaving the house. Did I just plug ebay?..............yes I did.

Those 5.6 ohm sand block resisters measure about 9 ohms, so I'm going to measure every related part. and also my 20 transistors with an appropriate tester and hand select the best pairs. Anything I should pay particular attention to on the board?

[Silvertone Jockey]

OK got all of the the new transistors, a new matched set of Mesa 6L6GC's and a week off from work to fix this thing and a new to me 7 string ax. Yee haw!... time to get busy

[Silvertone Jockey]

It's alive! yes it lives............... ahha ha ha ha ha

Mr Peavey's Frankenstein amp creation sounds pretty good clean, can't say I'm real impressed with the overdrive sound, but I was playing a 7 string guitar I just got that needs a setup job and I'm not used to it yet, so I'l defer judgment till after I check it out with one of my regular wench's. That 7 string with the neck humbucker running in split mode has a very unique sound, also have to check out if this amp is part of it's cool tone and plug it into my regular setup for comparison

Later though time for bed

[Silvertone Jockey]

Could the moderator please edit the title of this thread for spelling? VHT -->VTX

TIA

[Silvertone Jockey]

I asked the question 'how does a Classic VTX sound? I can now answer the question myself.

To me it sounds absolutely amazing. Hands down the best amp I've ever had. I haven't even got it dialed in yet, but once I realized the way I needed to play in order to take advantage of it's unique qualities, it got my adrenalin going. Somewhat raw tone that cuts like a knife with great sustain but something else I've never heard before in other amps. Its hard to describe it. It's the way the sound is like modulated* by the amp. It makes the instrument sound more alive but also requires greater precision when playing which is good because it forces me willingly to step up my game

That tone the 7 string had was the amp not the guitar. A special thanks to my friend who brought it to me because it was out in the weather in some guys back yard, DOA already suffering from rain, a kicked in grill, torn speaker cone, sun bleached and missing the knobs.



** "modulated power supply" indeed!

[R.G.]

Thanks for the replies, working so no time till the weekend but I read the other thread, and will do some testing this weekend. This amp reminds me of the many (triode, tetrode amps are grid driven usually) RF amps I have worked on which are almost always cathode driven, but the circuitry on those is usually much simpler. Ground the grid. Ground the cathode through a choke, give it some high voltage and a signal to the cathode and that's basically it, minus the tank circuit.

That's so the grid-plate capacitance is grounded, not amplified by Miller effect. You get a lot more high frequency RF response. Takes a high current drive to the cathode, though.

[R.G.]

An interesting side note is that it's common for some RF amps using the same 2 x 6L6GC tubes to achieve a hundred or so watts RMS, but the plate idling current is kept to extremely low levels with a variable bias circuit, which does cause some (usually ignored) distortion

In RF work you get to use tuned circuits to filter out the harmonics. The 1000 to 1 range of audio is massively larger than the range of RF bands. So you can run Class C, blipping the tubes on and off at the right frequency and keep tube dissipation low.

[Silvertone Jockey]

Hi, just found this thread and thought I'd share a few thoughts.

I've just bought a used Bandit 65 (not even had it delivered yet) and have set to work planning mods. I like the Peavey clean tone so don't plan to do much there but agree with many that the distortion leaves a lot to be desired. Even in when it was first designed it sounded crappy among its peers.

I've taken a look at the schematic for the amp that was provided in this thread:

http://music-electronics-forum.com/t16210/

and sketched out the gain/distortion stage in a simulation tool:

[ATTACH=CONFIG]31633[/ATTACH]

Ignoring the diodes I have circled in red. This is pretty much as per the schematic with the saturation knob set to its minimum and the Lead Pre knob set to its max (so max distortion achievable with minimum saturation). I used this schematic to run a simulation of the circuit with a 1kHz sine wave input signal.

The waveforms below give the actual output signal and an FFT showing the frequency content including harmonics out to 20kHz:

[ATTACH=CONFIG]31634[/ATTACH]

Looking at the signal this is an attempt to mimic overdrive rather than distortion where no signal clipping occurs but instead we see slew rate distortion. Essentially, gain is applied to small signals but not larger signal. It's a form of compression really. Seems quite neat until you check the FFT on the left. Basically, we are still seeing distortion totally dominated by odd order harmonics (3, 5, 7 etc). So, pretty noisy, not very musical and certainly nothing like the even order harmonics that you get from a nice Valve amp in break up.

Spin the saturation knob to the max and things get 'worse':

[ATTACH=CONFIG]31635[/ATTACH]

I say 'worse' because this is now just a classic distortion signal. Hard clipped top and bottom and totally dominated by the odd order harmonics again. This is actually okay if you are looking for a true distortion tone (it's more or less what the Boss Ds1 does). But it's a bit disappointing since, sonically, it's not really much different to to the tone with the saturation knob at its minimum.

Are there any easy mods to fix this? probably yes. I should add that I hae not been able to try them out yet but below is my own plan for my used Bandit 65:

Mods to the schematic:

[ATTACH=CONFIG]31636[/ATTACH]

All I have done here is to add two additional diodes to the feedback section (at the top) and one to the hard clipping section (at the bottom). The aim is to produce asymmetrical clipping which accentuates the even order (more musical harmonics).

What does it do?

Overdrive (saturation at minimum):

[ATTACH=CONFIG]31637[/ATTACH]

As you can see, the waveform is now asymmetrical about 0V and the result is that the dominant harmonics are now the 2nd and 4th, which are a lot easier on the ear. The dominance of these even order, lower frequency harmonics 'should' also lessen the buzzy wasps in a jar tone of the amp.

Distortion (saturation at maximum):

[ATTACH=CONFIG]31638[/ATTACH]

Again, asymmetrical waveform about 0V and greater even order harmonics. That said, it's meant to be a saturated tone so hard clipping is important so I have only added the one extra diode in order to maintain the 3rd and 5th order harmonics dominance.

The diodes I have messed with are CR3 (three in place of one) and CR5 (two in place of one). You can just use the same 4148 diodes already in the design.

Want to go further? Replace each silicon diode (CR3, 4, 5, 6) with a pair of germanium ones (like OA90s). Of course, to maintain the mods described above CR3 would be replaced with 6 series germanium diodes and CR5 with 4 series germanium diodes. Germanium diodes can/do make a difference in clipping circuits. I have used them to good effect n the hard clipping stage of my Boss DS1.

Anyway, possibly a bit premature given that I have not completed the work yet but thought I would share.

Cheers,

iep

I wish to pursue the same types of circuit changes described in the quoted post from a different thread above, but on a Peavey Classic VTX instead. Can anyone please point out the comparable part numbers (if there are any comparable) on the VTX schematic?