Tube Amp Archive

Hmm, OK. The NFB loops that I have seen don’t have the cap, so I thought it might be something different. Thanks.

And is the 470K/680K voltage divider critical? I have put a pot between the 470K and 0.003uF cap so that the 470K can be varied as high as 2.25M and it gives a wide variety to the tone. I’m not sure if varying the resistance that much will cause any problems for the phase inverter tube.

Thanks for the replies. I should have been more specific and mentioned that I have changed all of the caps, e-caps and coupling caps. They all looked like they were shot. I used the same values as shown in the stock schematic. Preamp plates are around 145 VDC, and the mixer tube plate is 166 VDC. Th...

I’m converting a Stromberg-Carlson AU-33 for use with guitar. The circuit is mostly stock but with new e-caps and new input jack and grid stoppers on the preamp tubes. Sounds decent as is but the tone controls don’t do a lot. As shown in the attached schematic the Bass pot comes before the Mixer tub...

Have any of you used the Orange VT-1000 or MKII tube testers? Are they considered to be good units for checking the remaining life of a tube and for matching?

Furman makes one, but very affordable?

https://www.amazon.com/Furman-P-1800-AR ... B001UNPCCM

Thanks everyone for the replies.

Great, thanks. Yeah, CC and about 15 years old so I may go back in and change them at some point.

In a push-pull power amp with grid stoppers, is it necessary or beneficial to match the resistor values? The reason I ask is I have a Marshall 18W clone with EL84s that has 5.1K grid stoppers, but they have drifted to about 6.4K and 7K. I'm trying to decide if I should change them or it's really not...

Thanks. The relay circuit is isolated from the chassis.

For a 6.3V heater winding that also powers a dc power supply for relays, can the center tap be elevated to the power tube bias voltage (29 Vdc)? Or, will doing that be a problem for the relay power supply? The relay board is by Hoffman and rectifies 6.3 VAC to 5.0 Vdc Apologies for the hand drawn di...

Looks like the fuse holder is not wired?

Edit: I see that the mains fuse is wired, sorry.

It’s personal preference, but blending the bright and normal channels is best for me. One channel is too bright and the other too dark, but blended they work great together.

I have used Miec 600V electrolytics in several amps over the past five years or so and they seem to perform fine. Scroll down near the bottom of the price list below. The 30uF value is about $8 plus shipping. I think there is a seller on Amazon now too.

https://www.justradios.com/cart.html

FWIW, I tried both of my AO-43 amps through 8 Ohm and 4 Ohm cabs. A single speaker at 8 Ohms and a pair of the same speakers in parallel at 4 Ohms. They both sounded fine with either load, and the transformers did not get hot. I did notice a slight drop in volume at 4 Ohms. I guess that is to be exp...