DIY Valve Guitar Amp Head

[Simon Barden]

It sounds like you may have more than 50W with the voltages present if you use the bigger transformer. Presumably the output transformer is OK for a bit more power?

[DrNomis_44]

It sounds like you may have more than 50W with the voltages present if you use the bigger transformer. Presumably the output transformer is OK for a bit more power?

I was thinking of using the existing output transformer that was installed on the chassis, but seeing as I opted to buy a new power transformer, I decided to go ahead and buy a new choke and output transformer as well, the new output transformer is rated for 50W output, has a 3,400 Ohm CT Primary and is rated for a primary winding current of 318mA, one of the power valves Evatco suggests for the new output transformer is the EL-34, since the KT-77 is supposed to be a drop-in replacement for the EL-34 they should work fine with the new output transformer, I'm not sure what the primary winding impedance of the old output transformer is, but it is designed for two EL-34 valves, as far as I know it still works perfectly well.

Most likely I will get more output power with the JJ KT-77 valves since they're supposed to have more headroom than the EL-34, I was doing a search on google for information about the normal biasing current for JJ KT-77 valves, and it's about 30 to 35 mA per valve, an EL-34 can dissipate up to a maximum of 25W whereas a JJ KT-77 can dissipate up to 28W, I've heard some amp techs say to adjust the bias so that each power valve dissipates about 70% of the maximum power dissipation, some say to adjust the bias so that the power valves are dissipating 60% or 50%, I added two 1 Ohm/5W cathode resistors to each of the octal sockets from pins 1 and 8 to ground, doing this allows you to measure the current flow through each of the power valves with a multimeter, the 1 Ohm resistors convert the current to milivolts, so, I should adjust the -V Bias to read from .030 to .035V across each 1 Ohm resistor, saves doing some calculations.

[DrNomis_44]

Update:

Got some more parts coming from Mouser Electronics in the post today, hopefully I'll be able to finally finish making the main HT power supply tagboard, as well as the power supply for the Vactrols that does all the channel-switching and muting.

Got some bills to pay this fortnight, but next fortnight I'll be able to start ordering the new transformers and 10H choke.


Update2:

These just turned up in the post, some 3W Metal Oxide Resistors, two electrolytic caps, and one 160V film cap.

[DrNomis_44]

Update:

Recently managed to make some more progress on building this amp after I ordered some stuff online from Evatco, I've got all the valve sockets wired up to the tagboards and pots, all I need to do to complete building all the circuitry is to order five new vactrols online from Evatco, then all that's left to do is order the new power and output transformer, choke, and a full set of JJ valves, plus some more 600V DC rated vintage-style cloth covered wire, and then I'll be able to finally complete the build process before doing all the testing/debugging/biasing, and then some play testing, will post some links to some demo recordings of the amplifier once I know it is all working correctly and sounding good.