6V6 to EL34 conversion in 20W plexi

[pjd3]

Hi there,

I built a 20 watt plexi which is great and has been my gigging amp. I primarily use it as a platform for my pedal board that outfits a few Runnoffgroove Tube-to-Fet conversion pedals (for marshall and Vox type tones) and finalizes with a chorus and reverb pedal (I hate playing without reverb!)

Being as the amp is acting as a pedal platform most of the time I'm looking to convert the output to EL34 since I also acquired a pair of new Winged-C El34's plus a quartet of Mullard Blackburn fx2 El34's that were on an old Leak hi-fi amp. The nice score of tubes plus believing the amp could benefit for having a more stout output section I'd like to try converting it. Not a big job, just needs to be done right with all critical things considered.

I found a 6A 6.3vac tranformer at Mouser that could be implimented for the higher filament current of the El34's but, I have to wonder if there could be a big shift in voltages in the HT and 5V rectifier voltage of the secondary in the existing power transformer. I don't know, theres just this impression that PT transformers were possibly designed needing to consider the total winding ratio of primary to all secondary coils. Does anyone know if that is true and that all the secondary coils need to be loaded in order maintain some kind of "coil inductance" value of the power transformer? If thats the case, I guess I could consider using the 6.3vac coil of the existing power transformer for only the EL34 pair, or only for the preamp tube filaments, share the filament load in some fashion or another.

What do you think are the important considerations for addressing the increased filament current? I'm trying to avoid the extra cost of getting a new bigger power transformer. That would be yet another home finance battle to win!

thanks for your input,
Best,
Phil Donovan

[pompeiisneaks]

An extra filament transformer would cover that, for sure, but you'd also need to get a different output transformer that's specced for EL34's
6V6's like about 8 k for their OT I think and EL34's like around 6.6k so there'd be a mismatch, I'm sure the tubes would work but at less than optimal levels.

You might also need still a better power transformer if the current rating isn't enough for the EL34's. 2x EL34 may take up to 80mA of current, per, or 160mA and 2x 6V6 may be around 90mA ... so just make sure the main B+ has some free room there.. those are just datasheet guesses, your tubes and setup will vary I'm sure based on B+ and other variables...

~Phil

[pjd3]

Thanks Pomp,

The OT in there now was speced for a 6L6GC pair, so I put the 8 ohm speaker on the 4 ohm tap in attempt to reflect a more proper impedance for the 6V6 pair. I was willing to see how that OT would work with an EL34 pair, thinking that it is somewhat in the ball park for the EL34 pair's impedance. Ha, that would be nice.

And yes, plate current, you bring up a big point. My only saving grace may be that I use this amp at a low to moderate volume for clean and pedal platform. How much voltage swing I would require from the amps application I can't be sure of right now. It looks like I would need to be ready to up the PT if its not living up to the EL34 pairs current requirements. I just got my first home oscilloscope for Christmas so will finally be able to look at voltage outputs in the amp under different conditions. The minimum I was hoping to expect was a little more openness and headroom and for some reason I was expecting a more mid frequency focused sound. I really have no justification for the last "wish" but it just always seems that 6V6GT versions of amps that also come in EL34's sound a bit brighter and thinner to me. That may just be an accident.

I'll just need to look at more EL34 circuits and see what else ought to be changed, perhaps grid stop resistors. My amp seems to really be a plexi preamp with a sorta Deluxe reverb output section - but maybe not for long!

Thank you,
Best,
Phil Donovan

[Phil_S]

You can probably get by with the OT as you describe, but the PT, if spec'd for a pair of 6V6 is a no-go due to the plate current requirements. So you might as well solve the plate, filament, and voltage matters by getting the proper PT. If you are buying a spendy PT, why not just go all in and build a proper 50W or 100W plexi? If you have a quad of xf4's you really owe it to yourself to do the right thing by them. And spend a little extra for good ear plugs, too.

The 3rd Phil!

[pjd3]

Hey 3rd Phil S, thanks for your input.

Yeah, I'd say you are right about the PT. I may just have to see about an upgrade there. My just justification was that I'm playing small bars and clubs and wouldn't need a lot of extra current since this particular amp would be relegated to clean sounds, rhythms and chorused strumming and arpeggios, and then my next project, a modded JCM800 2204 would be for the hard rock and lead sounds. But, no doubt about it, the amp will be looking for more plate current.

A little "Phil" story here for ya - I am actually Philip J. Donovan the 3rd, and my son is Philip J. Donovan the 4th. When my wife and I were deciding on naming him after me, my Dad says, "No Phil, don't do that, things will get all mixed up with the IRS and you'll get all kinds of hassle and trouble. I thought my Dad was being a bit over the top with that but, one day, I went into a hospital for something and when they looked up my name for billing purposes, they said they couldn't do anything for me because I had outstanding bills. Well as it turned out, the name they had was in fact my grandfather who had died many years before that.

I really have to wonder if I'll be telling my son the same thing if he ever names one of his sons "Philip J. Donovan the 5th" ! NOOO, DONT DO IT !!!

Best,
Phil D the 3rd

[stephenl]

IMHO, ideal tube to OT impedance matching is relative to optimizing power/bandwidth/distortion...mostly for HIFI. For guitar amps (narrow bandwidth, coloration welcome) not as critical.

With a 400v HT and an 8K primary OT, you'll have a 20 - 25watt PA that can use most of the typical power tubes and sound good...as long as your design can safely bias the tubes within their safe operating range and your power supply has a current rating appropriate for the operating class and can power the heaters, you're ok.

The 400v / 8K OT design is typically what I build. It sounds best to me with KT66's (JTM45). I've put 6V6's, EL34's, 5881's, KT88's, 6L6's in it all sound good, but different. KT88's just stay super clean and don't squish .

I pretty much always put in PowerScaling.

[pjd3]

Hi Steve,

I am very intrigued by power scaling, from the few utube vids I've listened to - still heard much of the critical elements of the desired sound at conversation volume. One day. I"ve only built 2 basic amps (Fender and 20w Plexi) but am ready now to delve into these kind of amps but with increasing versatility. Theres an Australian guy (Headfirst I think) that has a few offers for mods and "upgrades" for various Marshalls. Thats probably next for me, and then once I've had some success with those, then take a look at power scaling as a volume/tone solution. Being as though I still have 4 dependents I can't get too greedy with money (although I sure wish I could) So, one little step at a time for me.

I've seen what appears to be comprehensive Power Scaling solutions at London.

I was looking at Edcor just now and see power transformers for around $100 that seem to provide the electrical specs for a 50watt plexi upgrade. For my up and coming JCM800 2204 project I'm looking at a HV Fet boost, No loss fX loop, Clean/gain channel relay switching, some Diode clipping (thought I had no interest but in the vids does add what I consider a useful tone texture for certain apps, plus a number of panel mods, probably/maybe adjustable NFB, cold clipper variety switching, maybe switchable 1st stage plate resistor, and build one those attenuators found at Marshall forum. That right there would be a huge progression in my personal amp building history.

So thats my plan, now the hard part - sticking to it.

Thanks for stopping by Steve, would to hear one of your amps some day,

Best,
Phil Donovan

[pjd3]

What do you think of this, I just remembered that I have an Edcor power transformer mounted on a board set up as a "tube-amp-design-station". Here are the specs

Edcor XPWR096

335-0-70-335 @ 200mA
6.3vac @ 4A
6.3vac @ 4A
6.3vac CT @ 2A

I did do the math to insert a .68 ohm power resistor in series with a GZ34 filament and it seemed to work very well, put almost exactly 5 volts across the rectifier filament while the .68 power resistor sucked up the other 1.3vac. I was given the go that this is a viable method of voltage dividing to give the GZ34 its proper filament voltage, that professional companies even used this method to obtain the 5vac for rectifier filament. This seems like a good way of utilizing what would be a spare 6.3vac coil at 4A.

200mA appears to be plenty for cleanish pedal platform EL34 Plexi 50 watt so I think I'm good with this PT.

With an HT of 335-0-70-335 using the cool Mullard vintage GZ34 that was in a vintage amp I obtained, that should give me a B+ of (335 x 1.3 = 435.5vdc, approximately). I've noticed that the preferences for EL34 pair Plexi B+ and plate voltages have a very wide range of what folks consider the sweet spot, some liking the EL34 plate voltage as low as 350-380vdc up to 450-500vdc and some beyond. Some who like the higher voltages thing the lower voltages are too soft, grainy or squishy, and some of the ones that prefer the latter think that the high voltages make an amp that is too stark or sterile. I get that. My B+ and EL34 would be in the middle of those ranges so, maybe I'll strike some happy medium. Its worth a try to me. The only thing left to do would be to decide or experiment with node resistors for whatever preamp tube voltages I find the best for this amps application. But hey, where its just after Christmas (we spend a lot on each other!), I wouldn't have to get into "conversation" about buying, you know, high priority items like Power Transformers!
So, unless I'm missing something, I'll be moving forward with making a more properish 50 watt plexi out of a 20 watt plexi. The 20 watt plexi has been real good to me and I'm sure would continue to serve me well in many conditions. Its just having all these good 'Ol vintage tubes is too hard to resist giving a home. I'm weak. And giving in to the evil tube amp devil.

Thanks everyone, feel free to comment on what I'm in for!
Thanks,
Best,
Phil Donovan