Tube Amp Archive

Those blue and brown OT leads better be connected to pin 3 of the output tubes! Do you have any experience working on tube amps? They were on pins 3. I built a 5e3 last winter with a selection of the Robinette mods. It works perfectly. This is the first broken amp I've tried to debug. The main stru...

Found and fixed the missing ground. Reran the voltage tests and everything now looks a heck of a lot better except the power tube bias - and the power tube screens which were 170V vs a plate of 450V. Didn't take long to see that the reason for that is that the screen resistors are 470k , not 470 ohm...

Oh wow you are correct. I'll fix that and retest.

I checked all the preamp tubes in another amp and they all still work so we are on a roll here.

Yes, my bad, I now see that I noted 90V on one of the V2 plates. With the missing connections made I did a current limited startup (bulb) and that went well. With the limiter removed I checked volts and found 450V DC on caps A, B, C and D, so the voltages aren't dropping downstream. I assume the pre...

I used an 8 ohm speaker as suggested. Tested with tubes in, found there are no plate volts on tubes 1-3, it's a missing wire on the turret board between the 4k7 and 1k resistors. Should be an easy fix. Also the heater ground resistors weren't hooked up to the heater wires so that might explain the a...

Yes, apologies, I'm doing that and see what you mean. I'll pull the reverb driver only. Is it safe to power up with no power tubes in and no speaker connected?

I plan to use my current limiting bulb for first switch on.

I'm taking a look at a single channel AB763 a friend built years ago. It never worked well - noisy - and a tech recommended he replace some resistors and wiring. He did, and it stopped working. He put it in store for 9 years, and just gave it over to me to see if I can figure it out. He appears to b...

Even though the amp is not spitting out any blocking distortion, I temporarily added a switch with a 470k grid stopper for the PI just to hear what the amp will do with it. And honestly, at first I felt that there wasn't any difference by adding the resistor. But the following day with fresh ears, ...

Just built this amp as my first project ever. I decided to go full throttle on the diy and sorted 100% of the parts myself, without starting with any kit. I also used a blank chassis as I really don't like the facing-up style chassis and wanted all toggles on the faceplate. I finished it literally ...

Ok, next topic - sag. These amps are renowned for sag when pushed hard at high settings. Does this mean the B+ drops due to excessive current draw? Has anyone every measured that? If the B+ drops significantly it might be cool to have some sort of mod which lit an LED when the B+ drops below a certa...

... Da Geezer’s Bypass Cap attenuator was another interesting idea for cathode biased output tubes. It was very effective at controlling output power but the idea just didn't catch on. You can see both of these schemes on page 7 of my amp scrapbook . The bypass capacitor attenuation methods you guy...

Next question I have to answer is whether to mod the tone control so that I can use it as a high cut without losing all the middle - and hence overdrive - that the standard tone control appears to do (due to its massive 4n7 capacitor). This topic has been addressed both on the Robinette site https:/...

Update: I moved the 68k resistors to the jacks, wired each pair direct to the 12AY7 grids, and the DC has completely gone from all 4 inputs.

Problem solved. Thank you all!

For the record, here are the reading taken from each patch lead tip to chassis ground when plugged into respective jacks, tube in our out: 12AY7 out: Bright 1:188mV Bright 2: 13mV Normal 1: 124mV Normal 2: 9mV 12AY7 in: Bright 1: 169mV Bright 2: 11mV Normal 1: 102mV Normal 2: 8 mV As the condition e...

Low plate Voltage can cause Voltage on the Grid. What is your plate Voltage ? On V1 I have 130V and 121V on the plates. However, the DC on the inputs remains even if I take the V1 tube out. So I don't think it's that. Since I posted, I found a similar thread on here where the consensus was that the...