Tube Amp Archive

This is easy to do with a voltmeter by paralleling it across the Rk's or a series 1R. A current meter has to be inserted (or pre-inserted shorted and then un-shorted), not something I see a way to do using a single center-off DPDT. This seems like the easiest and most reliable solution. If you're d...

johnnyreece wrote: ↑Wed Jul 30, 2025 5:39 pm I'm sure someone much smarter than me can figure it out, but I don't know what switch will do it...

Edit: Best I can come up with works with an on-on 3PDT. If it was a center off switch, center off would truly be "Off"

. I’ll get me a 100mVDC meter. Seems easier

johnnyreece wrote: ↑Wed Jul 30, 2025 1:45 pm Wouldn't that link the cathodes? That's what I couldn't get my head around; keeping the cathodes separate, because the meter would link them.

I think so to. In the off position they should be tied together through the ammeter. 3PDT to isolate

Yes that’s my problem. I only have an ammeter (at the moment). Wouldn’t it work if a large bypass cap is out of circuit? What I got for suggestion was: Looking at older designs: Cathode->ammeter->resistor-> ground. Cap own ground. 1 Cathode 1 resistor + 220 µF cap node (bypass path) 2 Cathode 2 resi...

Hi and sitting bored in the car and contemplating a wiring for an ammeter in an integrated tube amp that is cathode biased. Might not use it since I will probably get an old west German percentage meter instead, that I probably would need help with setting up. But regarding this ammeter thing. In an...

Stevem wrote: ↑Sat Feb 18, 2023 6:48 pm It’s the mainly the screen voltage that sets the current peak, so that low 167 it what keeps that 450 from harming the tubes.

But would you benefit from that regarding tonality? what would be “better” with that setup compared to a more commonly used Vp of 350V and screens at about 315V?

🙏🏽 That's very kind of you. I want to be sure you understand that transformer may or may not work in the BR-3. Good luck. Just got the info. It’s a 1:3 winding ratio. The crazy thi with this amp are the voltages. It’s a PP 6V6 amp. PT 375-0-375, B+ 464, Vp 450V, screens 167V and cathode 12V. I will...

If you have such an amp it would be highly appreciated if you took the time to check the winding ratio... The problem is that I can't get to it without a major effort. However, I have some good news. I managed to find my newsgroup post (alt.guitar.amps 10/29/08) that documented what I could determi...

I believe Mercury Magnetics makes a replacement for the interstage transformer. Yup. Here: https://www.mercurymagnetics.com/products/?swoof=1&filter_make=gibson&filter_model=misc&filter_product-type=interstage-pi-transformer They are pricey and not specifically for the BR-3. I suspect the one for t...

Posted this on a lot of forums and need help. Stuck. Planning on an all metal PP 6V6 amp. Leaning toward a Gibson BR-3 (only 79 made). Instead of the loctal 7B4 I’m thinking of the octal equivalent 6SF5’s. My main concerns, due to lack of knowledge, is the PI and not to use a field coil speaker. As ...

Checked the frequency of the hum. 100Hz. Redid all groundings including grounded both OT at the bus close to first filter cap. Resoldered most of the connections and redrew some of the wires that carried the signal.
Hum went away. Happy

Please redo the test with the tubes in. Ok. Will do tomorrow. So now tomorrow. Measured with tubes in and on tha las filter node (B+ 273VDC) started at 0.91VAC and slowly increased upp to 1.2V On speaker jack 0.029 VAC. Did some experiments also. When touching the chassis the hum decreases a bit. S...

I made a mistake in a previous post. Since the PT has a heater CT I didn’t use the artificial CT on this build. Sorry for messing up info. The PT has a HV CT and the heater CT and both are connected to the negative node of the first filter cap as it is now with the hum. As I have been reading the ri...

First lets start off a check out session with this. Set your meter for low AC volts and note what you have for ripple voltage on the last filter node that feeds the preamp section. This would be a indication of the amp's 100 hz power supply noise level. Next go to the speaker jack and make the same...

1) will measure for real. Sounds like a 50Hz hum but even the sun has its spots. 2) yes. CT connects to the same point as the first filter cap connects to. 4) main safety earth connects to a single point in the chassis. Nothing else is connected to that place. One concern I have is that the power j...