AB763 Power Tube Mod

[mikeboone]

Where can I find the info to convert my BF Super clone to EL34's?

I want to experiment with that british tone without buying a new amp

[stokes]

Are you able to re-bias the amp?If so,just make sure pin 1 and 8 are connected on your tube sockets and add a 1k 3-5 watt resistor on your screens.

[drlowlow]

Another concern is that the EL34s will draw more current for the heaters. make sure to check that your transformer can handle the extra current.

[Bruce / Mission Amps]

EL34s need about 10-15v less negative bias voltage applied to the grids of the power tubes, right from the get go. Most stock Fender amps will not go that much lower unless you modify the bias supply.
However since it is a clone, this might not apply... there isn't anyway we could know.
Also... do not just ground 1 and 8 as told previously because almost all Fender amps (most clones too) have lug 1 and 5 connected with little 1/2 watt 1500 ohm resistors.
The bias supply and the actual driving signal feeds to lug 1!!
Grounding lug 1, by connecting it to 8 (which is grounded), will kill the signal from the phase inverter and short out the bias supply, blowing the power tubes.
For El34 use you need to move the signal wires and the 1500 ohm resistors off the sockets and maybe even replace the 1/2 watt resistors with 1500 ohm to 5600 ohm 1/2 watters ... and connect them only to lugs 5... float them straight up in the air. Reconnect your signal/bias wires to the other end of the resistor.
Most 6L6 Fender amps and clones use a little 1 watt 470 ohm screen resistor on the sockets.. EL34 amps need 1000 ohms and it has to be a 5 watt resistor, not 1 watt. Replace those too.
Then you can connect 1 and 8 for chassis grounding.

All this is a good reason to do a lot of research on this project first or take it to a tech who knows what they are doing and pay them to do it right.

[hasserl]

Besides all that, I don't think simply putting EL34's into your amp will give you "British" tone. It will give you Fender bf amp w/ EL34 power tubes tone.

[mikeboone]

Thanks Bruce... The amp is a straight up copy of an AB763 normal channel. I didn't bother with the reverb/trem as I use niether and it's just more opportunity for problems in my build.

I can easily modify my bias circuit to accomodate a different range of voltages.

Some of the early Marshall amps were very close copies of Fender designs with EL34 power tubes, thats kinda the direction I am headed.

It's all really about discovery and playing around. I have an AC30 and a BF Super that I gig with, so If I get this amp all tangled up with mods, I can always start over, ehh??

Thanks for the input guys...

[Bruce / Mission Amps]

Thanks Bruce... The amp is a straight up copy of an AB763 normal channel. I didn't bother with the reverb/trem as I use niether and it's just more opportunity for problems in my build.

I can easily modify my bias circuit to accomodate a different range of voltages.

Some of the early Marshall amps were very close copies of Fender designs with EL34 power tubes, thats kinda the direction I am headed.

OK... keep in mind that for the most part, the only Marshall amps that were similar to any Fender amps, were probably the tweed Bassman... you are doing an EL34 version of a AB763 circuit, way different.
I don't see any problem what so ever myself except those that I already stated.
You might find that you like KT77s better though.

[mikeboone]

Thanks again Bruce...

I will give it a go and let you know what happens. I suppose I need to do some more homework on early Marshall designs.

[mikeboone]

Tone Report

Besides all that, I don't think simply putting EL34's into your amp will give you "British" tone. It will give you Fender bf amp w/ EL34 power tubes tone.

Guys... I finally got around to doing the EL34 mod on this amp. Funny thing is it doesn't sound British, it sounds like the clean channel on my Single Rectifier.

Thanks for all the help.