Tube Amp Archive

Volume at the master decreases hiss tremendously when the gain is moderated for volume. If you play this amp at low volumes with everything else cranked it's all hiss. Crank the volume, and roll back the tones some. In a similar thing my bass is about 4, mids 6-7, treble 5-6, presence can be almost...

Well I spent quite a bit of time playing with this over the last couple of days. I scrapped the #39 schema I found on the web and started over. I have it working and the successive gain stages are cooperating with each other. Wicked gain by the time the last one is engaged. No more hissy mess, just ...

I just took a look at the Caswell 39 thread on Metro forum. There's almost 5,900 posts! Rather than read through that, I decided to scrap the internet schematic and draw my own. I've started wiring and will report back once I have something that works.

Thanks guys. I was going to start with the treb caps on the voltage dividers. They looked like an obvious source of hiss. Also will lower the gain on the two O/D stages. I'm running the preamp into a 6V6 SE power amp for purposes of tuning the circuit. Once I get it working properly I'll build it in...

I feel morphing old radio amps is the most fun of all amp design/building. A couple years ago I rebuilt an old RCA radio. I didn't use the cab, but rather built a 1x10 tweed combo with a Weber 12A100 alnico. As you can see I gutted the amp and only salvaged the chassis (with serious mods), PT, and O...

I've had limited success working with this design. I'd like to master it as I want to experiment with a 4 gain stage preamp. I thought the Caswell #39 would be a good place to start since there's a somewhat accurate schematic floating around on the web. The problem is when I engage the final gain st...

Follow up. I solved the microphonic problem. It was a bad piece of shielded cable I used for the 3" - 4" run from the vol wiper to the grid of the tone stack recovery triode. I use Belden 88641, but there was something wrong with this piece. Replaced it with a piece of regular wire and the problem i...

Been using the scope to look at signal path voltages under varying conditions. It's experimentation and education. Does anyone know what the "normal" gain range at V1-A and V1-B is for "typical" amp? Something like an old Fender where the fist triode is input gain stage, followed by the tone stack a...

Thanks, guys! Your replies are quite helpful. Enough so that I'm prepared to carry on, and select the values for my next build. This build is SS rectified.

I will order the Blencowe book on power supplies. It's one I don't have, but should.

I always base my filter cap values on what has been traditionally used in similar circuits. But I'd now like to learn how to properly size them. The two obvious categories are power amp and preamp. If the cap is only supporting a single dual triode tube, is 8uf sufficient, 16uf, 22uf, etc? As more t...

Problem solved. Thanks to your help!

The -VBias was too high. Once in the -12VDC range things settled down. I guess the 6G9 schematic with the -22.5 is not to be trusted with EL84s.

Thank you so much guys! You solved in minutes what I wasn't able to do in two days.

The EL84s will need something like -12V to bias correctly. I'm not an expert, but your plate / screen voltages sound awfully high to me for EL84s. I suppose the voltage will drop a bit once the bias current is flowing, but you might want to double-check. Andy My reference is the Fender 6G9 schemati...

I've tried both of my meters. One is a junk auto ranging Extech, and the other is a quality FieldPiece. No joy with either.

I just completed a Tremolux clone based on the 6G9 EL84 circuit. Note this is uses EL84s, NOT 6L6s. The only mods I made were to add a MV, tube buffered parallel FX loop, and variable bias circuit. The amp is built from the 6G9 schematic, but with the three mentioned mods. The Weber bias calculator ...

I did some unscientific experimentation and incorporated most of your grounding recommendations. What I didn't do was try each change by itself to see what specifically was the root cause. Here's what I did: 1) Wrapped the tank in tinfoil 2) Isolated both RCA jacks from the chassis 3) Ran the RCA ja...