6L6 to EL34 modification

[Macshaft]

I have just converted my Red Knob Fender twin to EL34`s . when I checked the resistors on the tube sockets , I was surprised to find a 1000ohm instead of the 470ohm shown in the wiring diagram for this amp . And the bias resistor that I lifted from pin 1 was 2200ohm instead of the 1500ohm I was expecting . I can`t get the bias voltage any higher than about 12 mvdc , and the amp seems to be running quite hot at very low output power.
The drawing I have is Fender 026059 . It looks right except the power tube resistor values are different . What gives?? Will I smoke this amp when I crank it ??

[txbluesboy]

EL 34s take a lower bias voltage than 6L6s. Where the 6l6 might take -50 volts to bias at 35ma the el 34 would only want maybe -35 volts. (Approximate numbers). You will probably have to adjust the value of one of the fixed resistors in you bias circuit to get this low a bias voltage from you bias pot. Don't know why it would run hot though. Terry

[Firestorm]

Sounds like somebody tweaked the amp, upping the screen resistors to 1K and the grid stops to 2K2. Make sure they didn't modify the bias supply. Stock, that amp has the dual-pot bias adjustment: one pot adjusts the actual bias voltage, the other one adjusts the balance. There's also high-power and low-power settings that affect the bias voltage, so make sure that Switch 11 (S11) is working right. Check the negative voltage at D305 -- you should have about -75vdc there. Then work your way upstream to see where you're losing your negative voltage.

[Macshaft]

That is correct ,it does have these pots for bias and balance - 25K each . I did not notice the S11 across the bias circuit , and now that I think about it ,I don`t think the low power setting works . At least i cannot hear any difference from Hi to low . The test points are across a 1 ohm resistor so my readings at TP282 - TP283 should be correct . R211 is a 100k in series with the bias pot , maybe a smaller value here??

Thanx Firestorm

[Structo]

I am interested in trying some EL34 tubes in my D'Lite 44.
Currently I am using 6L6GC's.

If I recall I used 470 on the screens and 3.3K on the grids.

Also on this amp, socket pins 1 & 8 are connected. (power tube sockets)

Does it sound like I could try EL34's without hurting anything?

I have dual bias pots that are 25K with 15K tail resistors.

[Firestorm]

Make sure your screen voltage isn't over 500V. EL34s won't like it if it's too high. You should probably change the screen resistors to 1K/5W. On an EL34 Pin 8 is the cathode and Pin 1 is the supressor grid so it's okay to have them connected (and grounded) but make sure there's nothing else on Pin 1. 6L6s have nothing on Pin 1 so it's sometimes used a tie point.

[Firestorm]

That is correct ,it does have these pots for bias and balance - 25K each . I did not notice the S11 across the bias circuit , and now that I think about it ,I don`t think the low power setting works . At least i cannot hear any difference from Hi to low . The test points are across a 1 ohm resistor so my readings at TP282 - TP283 should be correct . R211 is a 100k in series with the bias pot , maybe a smaller value here??

Thanx Firestorm

R211 and R208 (I think, hard to read) are the grid return resistors. The ones you'd tweak would be the load resistors on the pots. R213 and R21(blur). You'd increase them to make the bias more negative. But first make sure the pots are working right. Those guys have been known to go south at the drop of a hat.

[Macshaft]

I added a DPDT switch with 2- 100K resistors and 2- 120K resistors , Bias current reading at test points was the same as before with the 100k set but droped to less than 5mA with the 120K`s . rewired with 68K`s in place of the 120K`s and was able to measure +35mA and balanced at 0Volts .

[gahult]

Do you have a parts list with your schematic?
If not I have a nice legible one with the parts list I can post here for ya.

Gary

[Macshaft]

That would be great , I have a .gif dwg. , poor quality though . Very hard to read some of the stuff . This red knob never sounded much like a Fender anyway . Sounds much better with the EL34`s . lost most of that harsh sound .