Peavey Classic 30 Cathode Bias / Half wattage mod

[ianmarks]

Hello all. I’ve obtained a Peavey Classic 30 and am looking to do some mods. I’ve ordered some new preamp tubes and am installing a g12h30. I will be fenderizing the input and may mess with C4 a bit, but am going to wait on the tone stack after some of the “essential mods” are completed.

I am having a hard time deciding whether to mod the amp to cathode bias as I am interested in the half wattage mod as well. If I wanted to do the 15 watt mod (two dummy tubes, two modified tubes,) would there need to be additional modification needed with the cathode bias?

I've attached the mod pages below. I may also mention that Enzo's version has the 60ohm resistor replace with a 100ohm resistor in the cathode bias mod. I am under the impression that this value would likely be changed if the half wattage mod was applied.



http://blueguitar.org/new/schem/peav...0_cathbias.jpg

http://blueguitar.org/new/schem/peav...5_watt_mod.pdf

[Enzo]

Or pick one thing and do it. SOmetimes heaping too many features on something just gets too complicated.

Cathode resistors are chosen for the voltage dropped across then from tube current. Cut the number of tubes in half and you cut the current through the resistor in half. SO yes, you'd need to make that switchable too. 60 ohms is used in some commercial amps, but I felt 100 ohms was healthier for the tubes. But it was a judgement call, not some scientific exercise.

Other than it was fun to do, making the amp switchable cathode to fixed was not all that useful a feature. It mainly changed the dynamic a bit.

As to half-power, decide WHY you want to do that. If you are looking for lower volume, it won't be very effective. The difference between 15w and 30w is only 3 decibels. If you are looking to lose some headroom for breakup, meh, maybe some of that to be had. And then there is what I think of as hood-up chatter. That is where a guy opens the hood of his car, and him and a couple friends stand there discussing it. "Aw, I got dual headers, had to grind the manifold. y'know. And I replaced the carb with a Weber. Got Ceriani forks on 'er too." Then they all look at each other and no knowingly, and open anopther beer. So if showing it off is the goal, then go for it.

[Amp Kat]

^^^this and just my experience but the Vintage 30 isn't the best speaker for that amp unless you parallel it with another bassier type speaker. It's better than the stock marvel don't get me wrong but it is very midrangy in that amp especially the 16 ohm model.

[g1]

That is where a guy opens the hood of his car, and him and a couple friends stand there discussing it. "...Got Ceriani forks on 'er too."

lol Enzo, my old italian Harley dirt bike has Ceriani's (unfortunately not the preferred Marzocchi's).

[Enzo]

Hah!! I threw that little cycle easter egg into the car basket just to see if anyone noticed. I prefer the Mostacciolis myself.

And dual Canollis...

[ianmarks]

The g12h30 is not a vintage 30. Though there are a few folks who like those in the peavey classic series, I am not one of them. I've got a Hellatone 30 (broken in celestion g12h30) arriving tomorrow along with the preamp tubes recommended over at the Peavey forum.

My main reason for the half wattage mod was to save on tubes as apparently there is a way to replace the tube with a resistor...or at least something along those lines I read a ways back. I understand about the necessity for the impedance change. I need to do some more reading on it.

I do want to do the cathode bias mod, though. I've read that the classic 20 was biased that way and I had one a ways back with a 15 and loved it. Again, I am not educated on the subject, so this question may seem ridiculous. Is the variable bias mod only useful in the fixed bias mode or is it useful in adjusting it in cathode bias mode as well?

I also understand the blind modding syndrome...mod just to mod....but that's why I am holding off on the tone stack changes. I'll be tlaking to my buddies over a scotch, pointing out the rust stains on the tolex that I tried half a dozen remedies to remove and failed...I am hoping that is not a sign of things to come

[Enzo]

Cathode bias is not readily adjustable, the adjustable thing applies to a bias power supply voltage - as in fixed bias.


There will always be someone pipes up to say you could mount a big power rheostat or something to make cathode bias adjustable, but in the world of practical solutions, consider it stuck where it is.

Yeah, I guess if you run half as many tubes you will only have to replace half as many. But... Well, think of the savings if you only put three strings on your guitar. I don't know how often you expect to replace your tubes, but you only save like $25 every year or two.

[km6xz]

Why not play the Classic 30 for a while and learn to make produce pleasant music, and if it can't, get an amp you do like. Someone will like to buy it from you if you leave it stock. Modding can ruin resale value and usually makes the amp less reliable.

6BQ5's are the cheapest power tube around, cost of saving two tubes every couple years might be a false economy.

Mods are best done when you identify a specific trait you want to alter and do not care about resale value. The narrower the trait you are trying to remove or introduce, the better chance of designing an appropriate change that would be accomplish. The vague "I want it to sound better" is surely a bad bet. "I want to reduce power to 15 watts" is a doable goal. But that implies that you determined your sound goal was solved by 15 watts. That is doubtful that such a mod would make it sound better however, it could but there is nothing inherent in reduced power is going to make a 6BQ5 amplifier sound much different.

[Amp Kat]

I agree with KM6 that this amp is not the best to mod and I've done everything you can do to the C-30 that can possibly be done including reinstalling octal sockets and running 6V6's which sounded really nice by the way. I've done Steve's mods,Cathode bias/Fixed biased switchable with external bias terminals with bias balance and the whole nine yds and if you are going to mod this thing you better be "excellent" with PCB boards and a damn good troubleshooter because the jumpers will break on you and the traces are very cheap and damage easy. I found (the hard way) the best thing to do with the C-30 is to run it at 4 ohms (two paralleled speakers or 3 ) and put a stomp box in front of it that is transparent like the BK Butler Tube Driver and let it roll. Steves cap mod for the distortion channel helps alot but the others were not a big difference.

[Amp Kat]

The g12h30 is not a vintage 30. Though there are a few folks who like those in the peavey classic series, I am not one of them. I've got a Hellatone 30 (broken in celestion g12h30) arriving tomorrow along with the preamp tubes recommended over at the Peavey forum.

My main reason for the half wattage mod was to save on tubes as apparently there is a way to replace the tube with a resistor...or at least something along those lines I read a ways back. I understand about the necessity for the impedance change. I need to do some more reading on it.

I do want to do the cathode bias mod, though. I've read that the classic 20 was biased that way and I had one a ways back with a 15 and loved it. Again, I am not educated on the subject, so this question may seem ridiculous. Is the variable bias mod only useful in the fixed bias mode or is it useful in adjusting it in cathode bias mode as well?

That was a misquote sorry I meant to say parallel a V-30 with G12H30.

The reason for replacing the tube is because the heaters are in series and if the tube is missing it will not work. The mod will only give you a -3dB difference at most and Imo you may not even hear the difference as it won't be half power in terms of dB SPL. The bias will still be the same so they will burn just as much as with the mod less two tubes and if you do the Cathode bias mod be prepared to be changing them even sooner as depending on the Cathode Resistor chosen you could be approaching 100%+ dissipation wattage.

[ianmarks]

Some history...I've owned every incarnation of the peavey classic series except the real old ones that are black and have the stripes on side. 100, 50, 30 and 20. Have had all their extension cabs 115e, 410e had the 100 in a faux tweed full stack, even had a couple of the black tweeds. I think they are an incredible value and get the job done. I ended up moving what I had when I played an Egnater Renegade and still have it and a tourmaster cab, but no practice amp. I picked up the 30 hoping I could clean her up, get my kicks and move it if I needed to. I failed at the rust stain removal and have decided to keep it as a practice amp for the long haul.


So....I've decided not to mess with the tubes. Both km6xz and Enzo made good points and I am glad I posted! I've convinced myself that the cathode bias is something I have at least got to try. Mixed reports over whether the difference is perceived as "better." Only one way to find out...

In conclusion, fenderized input, cathode bias, speaker and tube swap....and a guilty pleasure mod....3 spring reverb pan....will start there and see where it leads before tinkering with anything else.

[Amp Kat]

If you are going to try it at least put a switch (power type) so you can tell the difference between the two because if you just make it Cathode biased you won't really be able to tell without a switch. It's a suttle difference but imo Cathode is a tad smoother but not very much at all. In other words I doubt anyone in the crowd is going to point at that amp and say " hey that thing is Cathode biased".

[Enzo]

In my contribution to the Blue Guitar materials, we did make it switchable. You can reach under ther and flip a switch and instantly hear the difference. It is subtle, not night and day. It doesn't really add to the complexity very much. I will say that the cathode bias mod biases the tubes hotter than stock, so if you want a real A/B comparison, you will want to add the bias adjustment mod so you can hea the fixed bias setting up to similar levels of the cathode biased. Otherwise it is apples and oranges.

The switch need not be large, the currents through the tube are small - milliamps - and the voltages are as well. I used one of those little red toggle switches.

[kleuck]

Why not use one cathode/resistor per tube ? This way it's easy to use non-paired cheap 6BQ5 (say, 6P14P from fleabay) by pairs or quad without adjustment (except the load of course)

[Bruce / Mission Amps]

Hah!! I threw that little cycle easter egg into the car basket just to see if anyone noticed. I prefer the Mostacciolis myself.

And dual Canollis...

I especially like the way my Bimbos work on my bike....