Classic 30 "tweed bassman" mod - first time modding...

[donbenjy]

I've just done the tweed bassman mod to my classic 30, and it sounds great, although it is a little harder to drive the power amp than I expected, but I'm currently in halls, so anything past 1 is pretty much a nono!

Anyway, the reason I'm posting is because I noticed the amp gets VERY hot really quickly. I've just done a test run for about 20 minutes and the chassis, particularly over the EL84s is almost untouchable. Now before the mod it did used to get that hot, but nowhere near as fast (at least from what I remember, I think it took around an hour, maybe less, to get to to that heat). I know that 2 power resistors were changed to bring up the preamp plate voltage, which I thought might have done it, but as I said, the heat seemed to be above the power valves.

It'd be awesome if anyone could offer an insight to why this is happening, and if it's likely to be dangerous? I've also noticed that it's possible to hear a small amount of sound with the volume at minimum, which I'm fairly sure wasn't possible before (though there was one time when I was messing around with oscillating pedals that did it, but that was seriously loud). You can't really hear harmonics or anything like that, but you can hear a tonal change when the pickups are switched, so there is sound (and I started out with oscillation before I plugged my Strat into an untested amp, which came through loud and clear with the volume at zero). This all might be hyperchondria to be honest, but I think it's worth mentioning.

At the minute I'm not particularly worried by these, but as this is my first amp mod then I don't want to take any chances and I want to make sure I've done it right. If anyone has actually completed the mod successfully, it'd be good to hear his experience.

Thanks guys!

DB

p.s. My regular forum would have locked it if I didn't post in reply to that other semirelated thread


edit: oh yeah, the mods at www.blueguitar.org for anyone that missed it!

edit2: pic of glowing el84s attached. I can't tell if they're red plating or just hot though... (or is that the same thing? ) That's actually the best shot I can get. It seems to be the "coil" that's glowing, which I would guess is the heater?

[stokes]

It isnt clear to me what you are describing as "a small amount of sound with the volume at minimum",but it is possible there is something oscillating and causing the tubes to overheat.The picture doesnt appear to show any red-plating in the power tubes,but an oscillation would make them work harder and get hotter than they would normally.

[donbenjy]

sorry, I just meant that turning the volume to "zero" doesn't completely stop any signal getting through! I checked the bias on the valves and they're showing 105mV on clean and 109mV on lead, which is apparently normal according to Alohas mods.

[stokes]

Do you mean 105ma's?If so that is way too high.What is your plate voltage?An EL84 should be around 25ma's at 350 volts.

[tubeswell]

1stly let me say that its "Ahola" not 'Aloha'.

Those tubes in the pic ain't red plating. If they were you'd see reddy-orangy glow on the outside of the big slabby bits of metal. The glow you can see beyond the ends of the plates in the pic is the tube heater filaments.

The preamp tubes'll run hotter after the tweed bassman CF mod (I did this mod about 12 months back - I think V2 runs at about 300V or so on each plate). But the output tubes shouldn't be. You'll only want 22-28mA on each EL84, if you run into the 30s they will start red-plating.

The lack of a complete volume cut of the level pot is because of the higher voltage in V2. You need a bigger value pot at VR3. Finding a large enough pot with the right taper is the next problem. I went up to 2M (and experimented with various tapers by using different resistors from the wiper to the ground) to try to find a big enough resistance with a good taper with sufficient strength to cut the signal. However I concluded that if you play on the clean channel, then what's the point of cutting the other channel completely?

[Steve A.]

These amps are very tricky to work on, and this mod is definitely harder than the other ones on my site. If the traces are not cut completely there could be leakage.

Also, I did the mod to my Classic 50, which had better access to the printed circuits for testing, etc. Although other people have adapted the mod for the Classic 30.

These amps run very hot stock which confuses the issue.

The only thing I didn't like about the mod was that the gain for the OD channel jumped up rather abruptly but I thought that that a lower impedance volume pot robbed some of the rawness in the tone so it was something I could live with. I did hear from someone who experimented with different values and came up with something he thought worked better, but I haven't tried it myself yet. (Hmmm... I bet that was "tubeswell"!)

Steve Ahola

[donbenjy]

cool, so the heat is not an issue then?

stokes: I didn't quite get that across did I? - I measured 105mV across a 1ohm cathode resistor, and using ohms law, we can say voltage measured here=current, so I have 105mA across all for cathodes (plus ~1mA at V3b) which equals roughly 26mA on each That's probably a little hot? I don't know how sensitive the bias needs to be? I went for a 22 multiturn trimmer so I've certainly got the ability to accurately change it. I've not actually put the trimmer in or replaced the resistors yet (just wanted to make sure the 1ohm resistor worked out) - would it be best to calculate the approx value to keep the current the same by comparing the original potential divider with the new one, or to set the pot at "zero" so the tubes are starved? Will that damage them?

tubeswell: yeah, I forgot to mention that I couldn't get hold the right sized pot for VR3 so I left it at 10k. Should've guessed that was part of it. To be honest I quite like the response of the 10k - would changing it open up the tone abit at lower levels as with passive guitar volumes? Anyone know of a (UK) site where I could get one? David Becker says that he used a 500k but was thinking about going down to 250k for less volume at lower levels. Does 1Meg step up really fast? Is the difference between 250k-1M really noticeable (seeing as I'll be going up from 10k anyway).

Steve: I went to town on the trace cutting (there was only one though - c2/r5 going to k1A to stop B+ being switched to ground). For the 1ohm bias resistor I just replaced a jumper and secured the resistor nestled in the inside of the joint made between the middle board and the tube board. I'll probably glue it in now I'm happy that it "works."


Thanks for the replies guys, I'm much more at ease now (to be honest it was probably 70% hyperchondria and me not wanting to blow myself up!).

I'm about to start the tone stack/boost mod, but (forgetting orange drops - I can't get them anywhere) I cannot find a 390pf cap in anything other than ceramic disk (to replace c11). I also accidentally ordered only one 0.022uf poly where I should have ordered 2 (c18 and c19). Should I bother replacing C11 - the value changes, but the quality of the cap is still crap. Also, would it sound weird if I replaced the treble and middle caps with mica and poly respectively, but left the bass cap as ceramic? Would it sound disjointed to have a good 2/3 of the EQ but a crapper sounding bass (or treble). I don't really want to pay £5 delivery for one component. Any suggestions? I've already been to my only local electronics shop and they don't even have silver mica! Also, what type of cap is best for C11? it doesn't say whether it should be an orange style poly, or a silver mica one, or something else... So yeah, is it a good idea to replace part of the tone circuit, or will that sound weird?

Any other "essential" mods that might be worth doing? Are C31 C32 C33 and C35 just to improve tone? How much of a difference would they make?

Sorry for the long post - one last question:

If I stick a new speaker in, what should I go for? I figured in true bassman style I could go for a Jenson, but I'm not too familiar with speaker nuances, so what might be a safe bet? The guy suggested a P12Q which is alnico, although it was a little out of my price range. I was thinking about a V30 because everyone knows them, but I don't really know what would be good!

Thanks again!

[stokes]

Didnt realize you meant 105mv for all 4 tubes.

[donbenjy]

Ok, I ran it for a little bit longer today, and everything seems awesome. When I initially installed the bias control and turned it on, the peak voltage on the cathode resistor got to 200mV before I could turn the pot down (stupid multiturn pots!) It was only for, say 10 seconds max, but is there anything I should be looking for in terms of tube damage? I presume if they're knackered they'll red plate? Or I'd at least hear a difference?

Sorry for all my n00b questions, but while I understand the basics of amps, I'm definitely learning from this experience!

Anyone got any info on the tone section? I've just put it completely back together, so i may wait until I can find all the "right" components before I open it back up! Any more tips for me?

[Steve A.]

Although I do like to use the polyester Orange Drop caps, you can also just high quality 630v polyester caps. I like stocking both polyester and polypropylene caps because the former can be a bit warmer while the latter can be a bit brighter. In some circuits going from one type to the other can be all that is necessary in fine-tuning...

With the film and foil composition of Orange Drops you can get parasitic coupling effects with the other components, which isn't necessarily bad- sometimes I think that the bass and mid caps in a Fender blackface/silverface amp can "talk" to each other a little bit...

With a roomy eyelet board you can move the caps around a little bit to fine tune the parasitic effects; on the Peavey Classic 30 board you are pretty much limited in where you can place the huge Orange Drop caps.

The Mouser catalog is great for looking at different caps, just to see what is available, although I really do like the ones stocked at the guitar amp specialty retailers because they get the ones that usually work very well.

As for speakers, don't be thrown off by the "tweed bassman" name- I used it because the mod gave the OD channel a direct coupled pair w/ cathode follower, just like in the tweed bassman. Other than that I think it might be more like a tweed deluxe cranked up to 13 than a tweed bassman...

One thing interesting about the design (unlike your typical 5F6A/Marshall-type amp) is that there is an extra gain stage before the tone stack which is driven from the anode rather than the cathode. The MV Marshalls added an extra gain stage before the direct coupled pair.

Steve Ahola

Disclaimer: statements in this post are strictly my own and sometimes have nothing whatsoever to do with reality!

[tubeswell]

(Funny - I can't seem to get the Quote function to work now. The writing window comes up with a totaly grey background and no quote text - and after I type into it, and press the submit button, it spits up with a message that I need to enter some text. )

Anyhow - on the VR3 pot size Qn, the reason I went to 2M was so I could add a suitable tapering resistor from wiper to ground (to further taper the pot response to get better control of the low-volume jumps) and still have about 100-220k overall resistance. 2M on its own would be useless.

I got so sidetracked that I did a series of excel spreadsheets on various pot tapering resistor combinations and plotted them out to see the effect the different tapers had. If I could load up excel spreadsheets to show you, I would.

[donbenjy]

Although I do like to use the polyester Orange Drop caps, you can also just high quality 630v polyester caps. I like stocking both polyester and polypropylene caps because the former can be a bit warmer while the latter can be a bit brighter. In some circuits going from one type to the other can be all that is necessary in fine-tuning...

With the film and foil composition of Orange Drops you can get parasitic coupling effects with the other components, which isn't necessarily bad- sometimes I think that the bass and mid caps in a Fender blackface/silverface amp can "talk" to each other a little bit...

With a roomy eyelet board you can move the caps around a little bit to fine tune the parasitic effects; on the Peavey Classic 30 board you are pretty much limited in where you can place the huge Orange Drop caps.

The Mouser catalog is great for looking at different caps, just to see what is available, although I really do like the ones stocked at the guitar amp specialty retailers because they get the ones that usually work very well.

As for speakers, don't be thrown off by the "tweed bassman" name- I used it because the mod gave the OD channel a direct coupled pair w/ cathode follower, just like in the tweed bassman. Other than that I think it might be more like a tweed deluxe cranked up to 13 than a tweed bassman...

One thing interesting about the design (unlike your typical 5F6A/Marshall-type amp) is that there is an extra gain stage before the tone stack which is driven from the anode rather than the cathode. The MV Marshalls added an extra gain stage before the direct coupled pair.

Steve Ahola

Disclaimer: statements in this post are strictly my own and sometimes have nothing whatsoever to do with reality!

ok, so it sounds like I should play about with different types to see what I can get. Best option in terms of not manhandling the fragile PCB (I've noticed if I change components more than once, the pad usually comes off, not to mention all the feeble jumpers!) seems to be bread/proto board so I can switch between different configs. I'm thinking poly, polyprop, and orange drops (and possibly ceramic, so I can A/B with the original tone). Hopefully I'll be able to emulate the orientation of the drop caps to see if they "talk" Do they only tend to interfere with only each other, or would it be with neighboring resistors too?

So far, after every change I've put the board back into the chassis, screwed down the tube sockets and all the jacks, and actually screwed the chassis back into the cab before I've tested, just to be extra safe. I have a few bad habits when I'm working with pedals, and I don't fancy being so sloppy with high voltages around! How do you guys tend to do it? Is it safe for me to have the pcb exposed and adjusting pots with the filter caps in such close proximity. I've never been shocked by a cap, and don't really want to, so what precautions should I take if I have to be fiddling while the amp is on? If I'm working with breadboard and swapping stuff around alot, obviously I'll disconnect from the mains, but should I drain all the caps too? I realize this might seem over-precautious, but not knowing how likely and how far a charged cap can reach I'd rather not find out

As for the bassman name, yeah I've played through a silverface bassman before, and the C30 did sound more like a deluxe to me. Any ideas on how to bring about even more power tube saturation? It sounds nice, but it's (obviously) loads harder to bring out a nice crunch tone. To be perfectly honest, the bluesy stuff is great, and I normally use pedals for distortion, but I do like to have a nice crunchiness at full guitar volume and just roll back for some cleaner sounds.

...
Anyhow - on the VR3 pot size Qn, the reason I went to 2M was so I could add a suitable tapering resistor from wiper to ground (to further taper the pot response to get better control of the low-volume jumps) and still have about 100-220k overall resistance. 2M on its own would be useless.

I got so sidetracked that I did a series of excel spreadsheets on various pot tapering resistor combinations and plotted them out to see the effect the different tapers had. If I could load up excel spreadsheets to show you, I would.

Sounds like fun. Could be another breadboard type crossover I think. Probably won't help me (I assume you're in the US), but where did you find a suitable sized shaft? All the ones I can find are 5mm diameter at the top, and the knobs fit 4mm ones!

[Steve A.]

With the Classic 30 you might want to keep experimentation down to a minimum, since the more you work on them, the more they screw up...

If you want to hear the difference between different caps its a lot easier to do that on an eyelet board in a Fender amp.

I brought up the subject of different caps since you mentioned not having a particular value and not wanting to pay a big shipping charge just to get an Orange Drop.

If one was going to work on a lot of Classic 30's it might be worth your while to make up a rig so that you could power them up outside of the chassis (the chassis is used for some of the ground connections as I recall).

Steve Ahola

P.S. Feel free to plug your pedals into the tweed bassman'd OD channel if you need more sustain and distortion- it ain't a Soldano!

[donbenjy]

ok, so just throw whatever good quality poly caps I can get in then?

Well, obviously I'm not looking for soldano type full distortion, but It would be nice to get a little more kick. Although I've only played my strat plus through it, which has gold laces...maybe my Tele will drive it abit more

Yeah, I get your point about it breaking more and more!

[Loudernhel]

Wow. I've been away from the amp world for a while and I find people are still doing those crazy mods on the C30, and that Steve has been hosting mine on his site for all this time!

Donbenjy, how did it work out?