The Amp Garage

I replaced the power tube sockets, power supply caps, bias supply caps, and added 10ohm 1% resistors to measure bias current at the cathode. I put in JJ KT77's - matched and burned in. Measured: Va = 475vdc Vs = 472vdc Vb = -40vdc - 37mA/tube(cathode + screen screen current) @ 70% dissipation: .7 * ...

What are the mosfets doing on the grids of the PI? Fixed bias scheme?

Thanks Ten Over, that is the correct version w/ the rectifier + single diode for the clipping circuit.

I'm currently waiting for a parts order to continue...

Has anyone tried replacing the stock clipping circuit w/ 24v zeners ala Jose Mods?

The stock resistor value for the bias feed is 220K for UK/EL34's. It's 150K for US/6550's. This amp has the 220K resistor in that position. Strange, the QC/Tester sticker on the end of the amp indicates it's for the US but its set up for EL34's. The resistor is identical to other 220K's used in the ...

Powered up with light bulb limiter w/o power tubes -ok Normal power up w/o power tubes - didn't clear fuses. Unloaded measurements: Va = 502vdc Vb = 500 vdc Vbias = -41 vdc at pin Bias range = -35 to -46 vdc I cleaned the carbon tracks off the socket. So maybe just a bad tube/tubes? I haven't done a...

Just looking on Tube Depot - a lot of backordered tubes...
What do you guys think of the JJ EL34's and KT77's ? I won't have a chance to power this up til tonight, but I think these have relatively high B+, >500vdc?

A friend asked me to take a look at his bandmate's 2205. He said he put in new tubes and it started blowing fuses... The schematic says that USA units came with 6550's - he had put EL34's in it. I'm not sure what was in it previously - need to ask. Not sure if it was re-biased. The speaker impedence...

Would it be worth considering "tubes up". I often wonder why "tubes down" are used in heads (that aren't redundant designs with a combo version).

Better heat management / reliability.

I've never played a Dumble, clone, or variant. Do any Dumble "ODS" players play with the overdrive circuit just set for edge of breakup and then push it with a pedal or just manipulate the guitar volume? Is this even do-able. I want to build something Dumble-ish - trying to decide with/without OD ci...

Any local feedback in the preamp?

Since there's no OD, would you put all pedals in front, or better to put in an effects loop for verb, delay?

That is sooo cool!

Well, the used Fishman battery I bought didn't hold a charge. So, here's my temporary solution... IMG_0515.jpg I have the same Fluence setup in my parts-o-caster and also had issues with the battery pack. The jack that comes with it didn't fit very well in the strat hole(compressed some of the sold...

Pete Millett sells these buffer boards on his ebay store. I haven't used this but have used some of his other stuff - well designed...

http://www.pmillett.com/A2_buffer.html

I'm thinking about building this one...
What is the purpose of the highlighted circuit segment - feedback/feedforward?