Uncle Dougs Supro Tremo-Verb... Need Help!

[sluckey]

In your first post you said "When I first powered up, the two in-series, 330 ohm/2w cb resistors burned up and the cb cap exploded." When that happened the tube and OT were also subjected to extreme current and likely damaged. Was your current OT in the circuit when this happened? If so, it's likely damaged.

When and why did you change the OT?

[Mjcopp]

Thats exactly why I replaced the original OT, (after the burned resistors and blown cap). The current OT hasnt been subjected to anything other than the backward caps.

[drew]

I have an  OT from an Epiphone Valve Jr on my junk pile that you can have for the cost of postage. 5W, 7.5k ohm primary; 4/8/16 taps. You'd want to use it with a 6V6.

[lego4040]

I already started putting parts on the side for this, I love Uncle Doug. I watch his vids and write his tutorial material down. I can understand what he is talking about the way he explains. I havent gotten a chance to read this thread yet but I hope you figure or have figured out your problem. Doug is old school with point to point and tag board. It took me a few to notice his grounding on the layout, certain tabs on his strips are grounding to chassis and thats where he lands the ground.

[sluckey]

By all means, take up Drew's offer on that OT. Even if it's not what you want to end up with it will tell you if your OT is the problem. Part substitution is a valid method of troubleshooting, especially with an elusive problem such as you have.

[Mjcopp]

Thanks Drew for the offer. I appreciate that. Message me with your info and we can work out the details.

[Mjcopp]

Drews' little OT arrived in the mail today. I wired it up and ....well boys, my amp now has tremendous volume! Thanks Drew! I had my doubts but you called it Sluckey. (I still cant believe I fried 2 OTs, thats just embarrassing.) The tone is ok but not great and break up occurs at very low volume 9-10:00.

Now I have a couple questions. I should now buy an appropriate OT rated for 15 watts SE and 5k ohm primary and install it before tweaking anything else, correct? In otherwords, I wont know if I need to adjust the bias until the proper OT has been installed? Drews OT is a 5watt with 7.5k ohm primary. Can I go ahead and try to adjust the cathode bias resistor value while I wait for a new OT to arrive, or just leave it alone until its installed?

Man, I tell ya, if this didnt work, I was ready to throw in the towel.
Thanks again guys. You have no idea what a relief this is.

[sluckey]

I don't think you need to adjust the bias. Those resistor values already have the tubes throttled back a bit. Just put a 6V6 in the socket and play it until you get the bigger OT. That little OT probably won't be happy with a 6L6 though.

[Mjcopp]

Greetings my ampaholic friends! Well, I have her buttoned up and ready to play. The tone is not quite what I expected, but very nice nonetheless. Quite sparkily and chimey, but not much for bass. I suppose this is inherent in the nature of SE design? I changed a few caps from .01uf to .022uf which helped a little. I also added a 250pf bright cap to the volume pot which really brought out the treble. Had an issue with tremelo motorboating esp. at high vol/intensity, but moving some wires calmed it down. Overall, it sounds beautiful to my ears... The only issue I would love to remedy is the lack of clean headroom. Clipping/overdrive starts at 9:30 on vol knob and sounds awesome when dimed...very crunchy. I tested v5 cbrs from 220 to 750ohms and settled on a 400ohm cbr that gave B+1: 354v, C/Pv: 340v, C bias vdrop: 25v, P current: 57mA, and P dissipation of 19.5 watts. This 400ohm cbr value gave the best tone and most clean headroom I could get. I wonder if replacing the v1 12ax7 with a 12aT7 would give me a bit more clean HR? Any thoughts?

Otherwise, I love the amp, I learned way more than I ever planned to and I cant wait to build another. I want to thank you Sluckey and Drew and all who helped a total rookie struggle through the most challenging project I've ever dealt with. You must tire of hearing it, but I never would have succeeded without your help! God bless you all.

Mike C.

[drew]

Are these things even supposed to have clean headroom? I don't know much about them other than the Jimmy Page connection.

[pompeiisneaks]

Are these things even supposed to have clean headroom? I don't know much about them other than the Jimmy Page connection.

I would doubt it, they're not designed to be a massive 'amplifier' in the preamp sense, more about slightly more than unity gain with all that tremolo and reverby goodness.  I'm sure that pegging them does provide some great drive, or even just at their max output but still clean drives the input of a guitar amp pretty heavily.

Like a tube gain pedal in a reverb/tremolo box :P

~Phil

[Mjcopp]

So... any ideas on how to increase cleans and calm down break-up?

[sluckey]

Don't expect too much from that cheap Supro. Build a Fender AB763.

[drew]

^ or perhaps consider one of the clean-oriented Dumbles/Dumble-derivatives documented over on ampgarage.com, if you want something less ordinary than a Fender.

[pompeiisneaks]

One of dumble's common mods was to do a LNFB on a triode to increase clean headroom.  To be more sure of where the break up was starting, you'd want to use a scope with a sine input, but, at a minimum, you could try on the last tube in the chain before the 6V6 and get a couple 22M resistors and some specific capacitor, like a .047uF and put them from the anode back to the grid, with Andode -> 22M, -> .047 uF -> 22M -> Grid

There's other variants, but that one is a smallish one.  You may want to try various types of capacitors there, but it will basically clean up that stage and increase headroom and reduce distortion.  On the tweedle dee deluxe he just used a 3.3M into a 2.2nF (.0022) cap as well.  you can toy with different values pretty easily by installing a 3 prong terminal strip, connect and use a few alligator clip leads etc, to connect a few differing resistor and cap values to get in the ballpark of what 'works' but doesn't impact the tone significantly.  Basically more resistance means less feedback and less impact, but there is also a specific frequency range targeted by the cap as well, so changing that will adjust what range is 'cleaned up' as well.

~Phil

[Mjcopp]

Thanks for the suggestions. I will experiment a bit with the nfb idea. Another problem I just noticed is a nasty speaker buzz only on certain notes - C & C#. Called Weber and was told it is invariably in the circuit. Any ideas where to check first?

[pompeiisneaks]

Thanks for the suggestions. I will experiment a bit with the nfb idea. Another problem I just noticed is a nasty speaker buzz only on certain notes - C & C#. Called Weber and was told it is invariably in the circuit. Any ideas where to check first?

That's one of those areas that can be very tricky to fix, sometimes it is a vibration in the cabinet, sometimes it is a tube that's a bit microphonic.  Try touching the tubes (careful they can burn, maybe with a glove) while playing the note to see if it goes away.  also try putting general weight on the cabinet to see if it goes away.

If not, it could also be not enough filtering, or other types of distortion.  Or just 'farty' due to the capacitors not being quite tuned for the specific setup you're using?  I think to guess more it may be better to hear the sound it's making in an audio clip.

~Phil

[Mjcopp]

Well...I determined the source of the "speaker buzz". It is the back panel chassis cover! Take panel off - no buzz, put panel back on - buzz. I tried playing around with the screws, inserting rubber washers to no avail. I'm beginning to think the panel is possibly reflecting the c-d note frequencies back into the chassis/speaker causing the buzz. I really dont know. But it apparently doesn't matter how the panel is fastened... if its on, there is buzz. Any advice would be , as always, greatly appreciated. Still have'nt gotten around to the nfb mod. Thanks again Phil for the help!

Mike C.

[sluckey]

Are you talking about a metel cover that attaches to the chassis or a wooden cover that attaches to the back of the cab? Show us a pic?

[pompeiisneaks]

Well...I determined the source of the "speaker buzz". It is the back panel chassis cover! Take panel off - no buzz, put panel back on - buzz. I tried playing around with the screws, inserting rubber washers to no avail. I'm beginning to think the panel is possibly reflecting the c-d note frequencies back into the chassis/speaker causing the buzz. I really dont know. But it apparently doesn't matter how the panel is fastened... if its on, there is buzz. Any advice would be , as always, greatly appreciated. Still have'nt gotten around to the nfb mod. Thanks again Phil for the help!

Mike C.

You could try adding some of that hvac metal tape to do double duty, act as shielding for the chassis, and it tends to help mute some of that kind of thing, or if not, any kind of tape on the back side may help break any vibrations building in.

~Phil