18 watt EL84 to 6V6 help

[Stratz]

Hi all. I'm fairly new to amp building. I've maintained and modded my tube amps (12 now) for the past 35 years but have only recently built any amplifiers. I've built a few amps, a few 5e3 Deluxe's and a Deluxe Reverb. Two were from Mojotone kits and the other two 5e3's were from spare parts hand wired using terminal strips so I'm not a complete novice.

I have a Mojotone 18Watt TMB kit with the 2204 JCM 800 master volume circuit and I'd like to use 6V6's in place of the EL84's.
From my experience the EL84 sounds great at lower gain but seem to fart out when pushed.

Other than enlarging the holes in the chassis for the octal sockets is there much more that needs to be done to use 6V6's?
Since the EL84 is a 9 pin and the 6V6 is 8 pin I dont know if they can be transposed.

I've already checked the power transformer specs and it should work just fine voltage and current wise.

I also have a set of Groove Tubes octal to noval adapters that I bought about 15 years ago to run EL84's in the new amp if I decide to leave the circuit stock until I'm certain that it works bug free.

Any help is very much appreciated.
Thanks
Rob.

[loudthud]

6V6s require different bias. Does the amp use fixed or cathode bias?

[Stratz]

Since there's no 50 volt tap on the PT it will be have to be cathode biased.
Thank you for the reply.

[Jazz P Bass]

The two tubes are not compatible, pinout wise.

The bias will also have to be taken in to consideration.

[Stratz]

The two tubes are not compatible, pinout wise.

I'm aware of this, the pinouts will need to be transposed. The bias will hopefully come down to the resistor.
I have 5W resistors from 100 to 330 ohm
Thanks

[frus]

Since there's no 50 volt tap on the PT it will be have to be cathode biased.

not necessarily, you can derive the bias voltage from hi voltage tap, like any 50W Marshall

[daz]

not necessarily, you can derive the bias voltage from hi voltage tap, like any 50W Marshall

Yes, thats what i did. Mine was a 18 watt i built with heyboers and i converted it to 6v6 with fixed bias taken off the HV. Heres my schematic...
http://music-electronics-forum.com/a...-2016-copy.jpg

[Stratz]

Thanks for the drawing Daz. The Mojo kit has a JCM 800 2204 master volume circuit. Other than that I might be able to do it with some help. I'm still learning to read schematics but this one doesn't look too complicated.

Would you happen to have the layout as well??

I'll have to research the Marshall fixed bias.

Daz, I see some switchable caps. What is the switching for?

Thanks so much for the replies guys!
Rob.

[d95err]

As an alternative to changing to 6V6 you could try adding a post-PI master volume. Turn it down just a little to ensure the EL84s are not pushed as hard, letting the PI distort more before the powertubes go into hard clipping.

[mac dillard]

Not familiar with the Mojo kit but if it is for a standard Marshall 18 watt TMB then the amp will be cathode biased. These are the changes I had to make on the 18 watt I built and changed from EL84's to 6V6's. Changed the cathode resistor from 130 (or 150) ohm which ever it is to 240 ohm or 270 ohm and check after fire up. Change the 47K (may be 57K) tail resistor in the PI to 27K. I would also change the 6V6 screen resistors from 100 ohm to 470 ohm. Good luck...Remember when you do this conversion the amp is no longer a "Marshall 18 Watt". It is a different beast. The EL84's are part of the 18 watt mystic.

[daz]

Thanks for the drawing Daz. The Mojo kit has a JCM 800 2204 master volume circuit. Other than that I might be able to do it with some help. I'm still learning to read schematics but this one doesn't look too complicated.

Would you happen to have the layout as well??

I'll have to research the Marshall fixed bias.

Daz, I see some switchable caps. What is the switching for?

Thanks so much for the replies guys!
Rob.

No layout, sorry. It was a 18 watt to begin with so when i radically changed the amp to a completely different preamp design i really didn't want to buy a new board and pars so i used what i have and therefore had to put a lot of parrts right on the tube sockets due to lack of room on the board.

As to the caps, one changes the tone stack;s treble cap value for a little different treble range and the other switches a treble peaker cap/resistor in or out giving it a tad less gain and a tad more top end when switched in.

[DRH1958]

From my experience the EL84 sounds great at lower gain but seem to fart out when pushed.

Farting out is blocking distortion which is largely a function caused by too large of value of coupling caps between the PI and power tubes. Reducing the value of these plus adding around a 10k grid stopper will many times stop this. Miller capacitance in pentodes is very low, so no worries about losing top end with adding the grid stopper. In my experience, the overdriven EL84 tube is the large part of the overall sound of a good EL84 amp. They add complexity to the otherwise buzzy preamp only type distortion. They have the highest voltage gain of all common pentodes, so add good amount of voltage gain many power tubes don't. Of course they add current (power) gain all power tubes should with it's Gm of 11,300 uMho. The tube itself doesn't cause farting out. It's the grid circuit as mentioned above and I've never heard farting out in any of my EL84 amps. It's my favorite tube for lower power amps. And ask how many original 5E3 Deluxe owners experienced farting out with the 6V6s. It's got .1uF coupling caps with the obligatory 1.5k grid stopper.

[Stratz]

Farting out is blocking distortion which is largely a function caused by too large of value of coupling caps between the PI and power tubes. Reducing the value of these plus adding around a 10k grid stopper will many times stop this. Miller capacitance in pentodes is very low, so no worries about losing top end with adding the grid stopper. In my experience, the overdriven EL84 tube is the large part of the overall sound of a good EL84 amp. They add complexity to the otherwise buzzy preamp only type distortion. They have the highest voltage gain of all common pentodes, so add good amount of voltage gain many power tubes don't. Of course they add current (power) gain all power tubes should with it's Gm of 11,300 uMho. The tube itself doesn't cause farting out. It's the grid circuit as mentioned above and I've never heard farting out in any of my EL84 amps. It's my favorite tube for lower power amps. And ask how many original 5E3 Deluxe owners experienced farting out with the 6V6s. It's got .1uF coupling caps with the obligatory 1.5k grid stopper.

Thanks for the great information DRH.
The PI coupling caps for the EL84's are 0.001uf. What size caps would you recommend for EL84's so it doesn't fart out?
Thanks

[DRH1958]

I would think those .001uF caps would be fine. I wouldn't think that low value would cause blocking by themselves. Many amps have either .01,.02 or .047uF for this duty. That's what I use and have not needed to increase the value of grid stopper. But also to consider is the rest of the coupling caps in the preamp as that can be subject to blocking as well. So the whole amp's freq. response has to be addressed. But I only wanted to make the point that a certain tube doesn't cause blocking distortion, it's a function of circuit design. Possibly the EL84s need different values of caps because they ARE different than 6V6s. EL84's are true pentodes whereas the 6V6 is a beam power tube as one example. And more times 6V6s are run at much higher voltages than the data sheet indicates.

[Stratz]

Thanks DRH. All of the coupling caps are small values. 680nf, 47nf, 22nf, and the rest are 10nf

I decided to stick with EL84's so we'll see how it sounds and go from there.

Thanks again!