1.5K gridd resistor 6L6/EL34 amp

[chocopower]

Hi!!

if i want to use EL34 valves in my Fender clone i need to change the 470ohm resistors in the power tubes for a 1k 5watt.. ...ok.

But.... what happen with the gridd stopers, the1.5k ones?... should i change them too?
In EL34 amps i use to see 2.2K to 5.6K....

Could i use 6L6 again without changes?



best regards from spain.... and sorry for my inglish...

[Steve Conner]

The higher grid stopper resistors will be fine for either kind of tube. However, the 1k screen resistors may cut your output power down to about 30 watts with 6L6s.

[MWJB]

Why would that be Steve? I've done this on occasion and a perceived power drop has never been an issue.

Chocopower, you also need to make sure pins 1 & 8 are tied together to ground. Many Fenders (& clones) use pin 1 as a mounting point for the grid stopper. In this case the stock grid stopper can be detatched from pin 1 and bent up vertically with your grid wire going to the top of the resistor. You might get away with 1.5K if your layout is good, I'd try it in the first instance.

[chocopower]

sorry...... dumb finger..

[chocopower]

Chocopower, you also need to make sure pins 1 & 8 are tied together to ground. Many Fenders (& clones) use pin 1 as a mounting point for the grid stopper. In this case the stock grid stopper can be detatched from pin 1 and bent up vertically with your grid wire going to the top of the resistor. You might get away with 1.5K if your layout is good, I'd try it in the first instance.

Hi!

No worry. I had read a lot about this mod before do it.

My amp is a Twin Reverb clon (6x12A.. and 2x6L6), Guyatone GA1050.

The power transformer had a 6.3v - 5A tap. Enought for 2 EL34?

The Bias resistor is a 470ohm, as in fender amps.
I know i need change it for a upper value... Which value do you think is a good star point?

Thats my amp Layout...




best regards from spain!

[MWJB]

6x 12A#7 = 1.8A of heater current.
2x EL34 = 2.6A of heater current.

So your total is 4.4A. You could be OK depending on how conservatively the PT is rated. To see if this is too much current draw, compare heater voltages with the full tube compliment & 6L6s, and then again with EL34s. If it drops below 6.3A, either pull any preamp tubes that you don't use (vibrato, Normal channel - they're 0.3A each) or consider an auxilliary heater transformer. If you pull the normal preamp tube you may need to rebias this tube if it shares a cathode resistor with the vibrato channel (or give each tube it's own cathode resistor.

Also check for excessive voltage drop on the B+ supply, more drop than say 20v with 6L6s, with tubes biased up could spell trouble.

I'd leave the 470ohm as it is and replace the resistor that is grounded from the bias adjust pot, bigger for less current, smaller for more current.

[chocopower]

Ok..

I,ll measure voltages before.

Thanks!!

[Old Tele man]

The higher grid stopper resistors will be fine for either kind of tube. However, the 1k screen resistors may cut your output power down to about 30 watts with 6L6s.

...6L6's are beam power tetrodes which typically have a plate-to-cathode (Ip/Ik) current "split" ratio of about 0.95 at idle and 0.90 at maximum power; while EL34's are true, 5-element, pentodes which typically have a much lower Ip/Ik ratio of about 0.80 at idle and 0.70-0.75 at maximum power.

...this larger "spread" of screen current means MORE screen voltage will be 'dropped' across the screen-dropping resistor(s) with EL34 tubes than with 6L6's, the opposite of your comment. It will be the EL34's that loose more power with 1K screen resistors than 6L6's in the same circuit, because their Vs voltage will "sag" more when pushed.

[Steve Conner]

Well, how come both amps I've built make 50W before clipping with EL34s and only 30W with 6L6GCs? I measured it.

The first amp used a plate voltage of 475, a screen voltage of 360, and a 1k resistor shared between both screen grids. Increasing the screen voltage to 475V brought back the missing power with 6L6s, but it made EL34s crank out over 60W and light their screens up like lamp filaments. I ended up installing a switch to choose 475 or 360.

The second one used 430V on both plate and screen, and a 1.5k shared screen resistor. I'm going to try reducing that to 470 ohms.

[Dave H]

“Well, how come both amps I've built make 50W before clipping with EL34s and only 30W with 6L6GCs? I measured it.”

That’s because EL34s wipe the floor with those wimpy USA 6L6s.

I measured something like 45W with EL34s and 35W with 6L6s in my amp with 1k screen resistors and 440V B+

Dave H.

[MWJB]

So are we saying it's the tubes, more than the difference between 470ohm & 1K screen grid resistors that account for the W difference?

[Dave H]

So are we saying it's the tubes, more than the difference between 470ohm & 1K screen grid resistors that account for the W difference?

Yes

[MWJB]

Well, that makes more sense to me. The EL34 socket conversions I have done have been to allow EL34 operation if the owner so desired at a later date, but always came to me & left me with 6L6s in place. The before and after performance never suggested a significant/noticeable power drop.

In fact, when we A/B/C/D'd a bunch of amps of broadly the same model (as much "the same" as Brown tolex Fenders get anyway, B+ all in the 495-510v range), the one amp with 1K screen grid R's seemed to have a little more poke, though there were other differences in the preamp wiring/voicing. One owner immediately ordered his amp be tweaked to match the apparently louder amp, I did the preamp job and replaced the 470Rs with 1Ks (as this is what the amp he heard had in it) - if there was a drop in output it was more than made up for in the differences in the preamp/voicing (6G12A to 6G12 tone stack, same cathode bypass caps, same coupling caps, same plate voltages).

[Old Tele man]

So are we saying it's the tubes, more than the difference between 470ohm & 1K screen grid resistors that account for the W difference?

...that I'd believe. EL34's can push more than their 25W plate rating.