The Amp Garage

FWIW while we are talking fuses, a slo-blo ("time delay") fuse will blow if you draw 2x the rated current for about an hour. It is rated to conduct the rated current indefinitely. Regardless of what the labeling on the old amps said, I think you may be asking for trouble with a fuse bigger than 2A u...

BTW, I got the bad sound to go away, in exchange for tolerating more gain than I wanted in the amp. I had to change the 6DJ8 top plate resistor to 100K from 15K, and that fixed it. I think the 6DJ8 was arcing. Can't figure why it was doing this only when the TMB channel was hooked to the PI. And I c...

apologies for the extreme poor quality recording, just did it on my phone real quick. Here's the story: this is an amp of my own design; I have built several of them and never had a problem like this. It has two channels, one a TMB channel with some gain and the other is a 6DJ8 cascode. This is the ...

Well, I don't think my problem is the screen grids, unfortunately. I put in different tubes, one old Sovtek, one old JJ, and the screen draw is in the same ballpark and the bad sound is absolutely the same. It's a harsh sudden brittle distortion like it's clipping somehow. Sounds like arcing to me. ...

22mA?!?!?

Anyway, I have some other EL84s kicking around, gonna try them and measure the draw.

Hey all- long time no post. Anyway, recent amp build I did (still on the bench) is using Electro-Harmonix (so they say) EL84EH tubes. Actually the tubes are labeled SOVTEK, so who knows, but according to AES, they are EH. Short story, the amp has a seriously bad distortion sound that I have previous...

I don't see how you'd put a metal-film cap in the bias supply. It just needs one 22uF cap to filter ripple. Depending on the vintage of the PR, it may have come standard with (likely) a 5U4 rather than a 5AR4/GZ34. A 5U4 will drop much more voltage than a 5AR4, and a 5Y3 is even less. With a 5AR4 yo...

BTW the effect on tone of replacing the filter caps with 80uF will be that it will sag less and generally feel/sound "stiffer".

Increasing the capacitance of the filter caps will change the tone noticeably. It has less of an effect downstream of the first one. I typically don't go much larger than 47 (aka "40" or "50) uF for the first filter cap in any amp, but 80uF is not off the chart. However, 80uF for all four is probabl...

Guys - I converted to LTPI by moving the trem circuit to V3B and using V4A+B for the LTPI. I don't use reverb so I pulled everything related to reverb recovery and drive, leaving me with an empty socket and an unused triode. I still have the stage after the make-up gain... so it's input -> tone stac...

Hey folks- I have a '78 Princeton Reverb that has been subject of many mods over the past 25 years and I never was happy with it (this is the whole reason I started building amps!). Finally I got serious about making it usable for me and made some changes to it last weekend. I had previously deleted...

FWIW, I am a huge fan of this cascode 6DJ8 design, and I am putting it in many of my production amps. I have been meaning to try it with a 12AU7. I think this is an unfortunate neglected gain stage design that has a lot to offer.

well ya know, I never build amps with negative feedback. customer special. anyhoo, this is the first amp I built with this particular OT, so I am sure I just got the wiring color code wrong. sounds good now.

later-

well, I fixed the problem by reversing the OT leads. Dunno why it didn't motorboat on the speaker but only the internal load, but regardless, it was NFB polarity causing the issue. This is the first amp I've ever built with negative feedback. I am not doing this to protect the output, BTW. The amp i...

OK, I have a low-watt amp I have built, and on the output I have a switching jack. When you unplug the speaker, it switches to an internal reactive load which consists of a speaker motor (no cone) in series with an 8.2 ohm power resistor. The amp circuit is sort of like a BF Princeton Reverb with no...