The Amp Garage

Nearly all amps I've worked on have bleeder resistors whose sole function in life is to drain the caps. But you never dive into an amp without measuring the caps, or having a schematic, and working out where the high voltages may be, sometimes the bleeders fail.

Funny you should mention 11k, I pulled some trannies from a Chinese EL34 PP stereo amp and they were 11k, I sort of gave up on ever using them in anything, they are about the size for 80W and the B+ in that amp was around 460V. I wondered how it ever worked

Interesting circuit. Looks like the Master Volume is not drawn correctly? It has true grid leak bias on the first 12AX7 stage of the "Gain" channel, i.e. no bias resistor. The clean channel would have to be interesting too. I built an EL34 amp with one channel and no tone controls and I like it. A b...

Ian, the S bench gm/mu tester (see http://greygum.net/sbench/sbench101/) is similar to what you are describing, but it has an adjustable DC supply for the filament voltage, a constant current supply for the cathode, and it lets the grid float. Dynamic parameter gm is measured at constant plate volt...

I have never read of an Edcor failure on the internet forums. If the filament wires coming from the transformer are melting then the problem is outside the transformer, not internal. Might be worth checking all your heater wiring, if it shorts again you might lose that transformer (or the heater win...

I agree with that, I have an old Taylor 45 tester, it works well but I can only compare tubes to known good ones. It is good for measuring the sections of preamp tubes too, and to select good balance between the two triode sections for LTP phase splitters for example. A friend had an amp that blew t...

Yes there is. A LTP with 820R/10K tail, 1M bias resistors, driving 2 x EL34 through 0.1uF caps with 4K7 grid stoppers, 220K to bias supply, 220 ohm screen grid resistors, 466V B+, blow most modern EL34's to bits possibly.

I've been working on an old amp that uses a 390 ohm resistor for some sort of soft start, but also leaves the screen voltage on the output tubes disconnected until you go from standby to run. Seems to work well. Quite clever I thought.

I think in the 60's an A pot was linear and C was normal log. Later they changed it to A = log and B = linear. Cameron, there is a more interesting schematic on the Vase website www.vase.com.au If you scroll down to the bottom of the main page to "Newsletter Back-Issues online for download", downloa...

I downloaded 5spice simulation software. It seems the range of bass is about 4dB at 80Hz and 8dB at 40Hz. It's a bass amp. Maybe I simply could not hear the change with a 6 string guitar. It looks like the bass control should work, and the amp has not been tampered with...live and learn. The "pull b...

I had an old Vase amp in for a lookover, I drew up this schematic. On final check, I found the bass control to have no effect. The "pull bass" pot switch does increase the bass, but the bass pot does nothing. I checked the pot, the cap and the resistor values and all seems fine, but no action. Unfor...

Just for info, if you want to go for something smaller, like 4 x EL84, Tubelab has a PCB and parts kit for $110 for a wonderful little amp, with tube rectifier. You have to source your own tubes, trannies, case and hardware. Definitely amongst the top contenders in the EL84 class. Pic of mine below....

bbus, I think you are measuring correctly. What you see is what you get. I have measured many OPT's and they don't always turn out to be the manufacturer's specs, but some are close. Even between different windings (say 4 ohm and 8 ohm) the reflected primary impedance can differ a fair bit. I have t...

100% true bypass. Unlike a tube screamer, it really sounds just like a Marshall when used with a clean channel of a tube amp, IMO. A good trick for the toolbox perhaps.

Oh I dunno, just listening to Black Sabbath, I feel for all the points made above. I had gear stolen, my only guitar and all pedals, not much fun.