6G2/5E3 hybrid

[shaun]

Heyo. Hope everyone is well. The slow down is sorta nice - more time to fool around in the workshop:).

I'm trying to reconfigure an amp out of an old Magnavox turntable. The amp was a 6V6 PP with a strange front end designed for an ancient turntable. The schematic was unavailable online, so I'm sorta wingin' it.

I want to rebuild it. I drew a schematic I thought might work. It has a 6X5 rectifier, which I thought I'd leave in. Maybe it would be smarter to use a 5Y3, but then I'd need a new tranny for the 5v filament.

Apart from the voltages being a bit low for the typical PP guitar amp, is there any reason why this schematic wouldn't work? The power section is from a 5E3, and the front end is sorta like a 6G2.

Thanks for any help, and may good health be yours.

[shooter]

If you build it as drawn don't see any smoke 

[DummyLoad]

the cathodyne in the fender plan works well when driven for clean: it's absolutely horrible sounding when pushed hard. the values in the attached will help minimize the distortion introduced by the cathodyne. it allows the final gain stage to swing the output tube grids into plate overdrive minimizing doubling and blocking distortion created by the cathodyne clipping.

i added screen stoppers. leo was saving a few pennies where he could. you don't need to. fourth filter added for 1st gain stage - lowers hum buzz as does the artificial CT: R25 & R26.

SCH expressSCH ver. attached for you to edit.

i'd add the 4th unused triode of V1 as a cascaded gain stage or strap in parallel with the 1st gain stage. perhaps slucky's tremonator & wobble V2 gain stage? hate to see a triode just sucking up heat.

--pete

EDIT: fixed notes and some parts alignment.

[Williamblake]

Appearantly the 6V6s where powered by a 6x5 before but i wonder if 70mA HT will suffice.

[DummyLoad]

Appearantly the 6V6s where powered by a 6x5 before but i wonder if 70mA HT will suffice.

apparently more common than i thought as well at one time. have seen similar hi-fi stuff with same setup.

personally, i'd drop in a 6CA4/EZ81 - filament winding should be able to handle another 400mA of load. B+ will rise according, but nothing to be worried about - one may have to tweak Rk for 6V6 to keep plate dissipation in check.


--pete

[DummyLoad]

6AX5 is another choice if want to stay with octals, but draws 1.2A - twice the 6X5 heater demand but has twice the available current. SSR or 6CA4/EZ81 are better options.


--pete

[66Strat]

With that power transformer, silicon diodes in the power supply would be a good idea, IMO. Another option for the unused triode would be to replace the cathodyne with a self balancing paraphase as used by the 5D3 Deluxe or maybe even an Ampeg SB-12. Sweeter cleans and better overdrive to the ears of this beerholder.

[shaun]

Wow - lots of great feedback. Thank you!

The EZ81 is a 9 pin small tube. I want to keep the old chassis and, as the 6X5 is an octal with a bigger hole, I'd have to start machining the chassis to fit the EZ81. Which I'll probably have to do for one reason or another eventually, but going down in tube size is a pita compared to going up in size. So I'll stick with the 6X5 for as long as I can.

The Spring Basic schematic looks very interesting on many levels, so I'll spend the day studying that. It's very close to what I was imagining - I figured someone must have done something similar at some point, so thank you very much for the schematic, Dummyload. Much appreciated. Did you draw that using .sch software? I can't seem to get it to work on my mac, but I wish I could - it looks great.

I agree - it's a shame to waste half a 12ax7, so I'll research the 6X5 and the trans to see if they can manage the extra current....although I'm guessing they can, but the 6AX5 might be pushing it. The 4th filter stage is definitely something to consider, so I'll try to work that into the chassis space I've got.

Thanks once again.

update - I just got JSchem to download and install, so it's all good.

[DummyLoad]

use an adapter for the 9pin to octal hole. no chassis machining and can be reversed cleanly.

as a guitar amp, the amp would likely sag heavily with the 6X5 rectifier.

https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=MiscHardware&ORDER_ID=224322637



--pete

[shaun]

Good point. I should probably have one or two of those adapters on hand in any case. Thanks Dummyload!

[shooter]

Hi-jackin, sorta  (using DL's schematic for reference )
My build is close enough to count 

DL-anyone, can you clear some fog on the cathodyne?
 I’m missing something;

Going from 1.5k to 10k on PA tubes (R17,18), Blocking distortion prevention or voltage divider, or..?

Is the cathodyne happy if I give it a distorted signal, but “swing it clean” to the PA grid.?

Bonus points;
 
The V.W. talking about your “R13” 1k
“I prefer the latter as Rb (R13) provides a little extra isolation from the power valve during overdrive.”

the “during overdrive”,  That’s the PA tube’s OD?

Thanks

Dave

All yours Shaun 

[DummyLoad]

Going from 1.5k to 10k on PA tubes (R17,18), Blocking distortion prevention

blocking

Is the cathodyne happy if I give it a distorted signal, but “swing it clean” to the PA grid.?

feed all the distortion you want - don't want the cathodyne itself to be the source of distortion. 

“I prefer the latter as Rb (R13) provides a little extra isolation from the power valve during overdrive.”

feed from cathode or from bias resistor/ tail junction? i prefer to tap UNDER R13 - i drew it that way to minimize confusion with a novice builder who is likely used to seeing the most common method. signal out tap under bias R also pushes Zout of cathode slightly higher and a tad closer to Zout of plate.

--pete

 

[shooter]

thanks Pete

from bias resistor/ tail junction?

ya, I don't currently have the bias R in, just cathode/R junction to grid R of PA, will add a bias R in.