EL34 vs 6V6 for 20W power range?

[SnickSound]

Building 15-20W Marshall circuit is something I really enjoy. I typically go with 6V6 and Deluxe Reverb transformers, and often cathode bias because why not.

But recently Marshall has been releasing a bunch of 20W amps running EL34s, as have Blackstar.

What's everyone's opinion on the best approach for Marshall tone at these power levels? The EL34 thing seems cool, I'd imagine sub-400V B+ into a higher impedance, perhaps with cathode bias too. But is it even still an EL34 at that point? Datasheet doesn't cover any such application.

So... run 6V6s the way they were meant to, or starve a pair of EL34s for "authenticity"?

[mwelch55]

I built a High-Gain Cathode Biased amp that has 360 VDC on the plates using 5881 and 380 VDC on the plates using 6V6.  If I install the 6V6 tubes, the plate current is at the max plate dissipation for that type of tube (that's OK for Cathode Biased).  If I install the 5881 tubes, the plate current is around 80-85 percent without changing anything else.  I get similar results with EL34.  It is very versatile and sounds great.  I personally like the Tungsol 5881 the best in that amp.  With 5881 tubes in the circuit, I get 20.4 watts into 16 ohms.

Mike

[SnickSound]

I built a High-Gain Cathode Biased amp that has 360 VDC on the plates using 5881 and 380 VDC on the plates using 6V6.  If I install the 6V6 tubes, the plate current is at the max plate dissipation for that type of tube (that's OK for Cathode Biased).  If I install the 5881 tubes, the plate current is around 80-85 percent without changing anything else.  I get similar results with EL34.  It is very versatile and sounds great.  I personally like the Tungsol 5881 the best in that amp.  With 5881 tubes in the circuit, I get 20.4 watts into 16 ohms.

Mike

Curious what transformers you're using?

One challenge whenever I look into building a mid-power EL34 amp is that PTs with the appropriate voltage tend to not have enough current handling on the 6.3VAC line to handle EL34s (1.5A a piece).
Since I'm in Canada, I try to stick with Hammond, so that makes the 290MX (AC30 PT) an option (6A on the 6.3VAC line!) but it's fairly expensive. Should be good for around 380VDC though, maybe 350-360 with a GZ34.
Or I go with their generic line and pick up a 270HX, but it's pretty much the same ratings and price, at that point it's more a question of stand-up or Z mount. Both are rated for 230mA on the HV tap, which should be fine for this use.

As for OTs... I'm thinking either a 6.6K primary (1750NA is 6K at 30W, close enough, it's a Z Mount though, which would be an issue for this build), or 4K (maybe stay with AC30 trannies and use a 1750T).
6K or above would allow easy swapping between 6V6 and EL34s, although I could just mismatch to turn a 4k into 8k with 6V6s.
Or I could just use a 50W OT at 3.2k and mismatch it all the time. Those are massive and heavy and might sound a bit too clean.

[66Strat]

IMO, the Hammond 270HX power transformer that you have selected is a good fit for this project. For the output transformer, the Hammond 1760J looks like a viable solution; thirty five watt rating with 4k primary windings and 4, 8, 16 ohm speaker taps. The 4K load would be a good fit (IMO) for cathode biased EL34s. A 100% mismatch would provide an ideal (IMO) 8k load for 6V6s. FWIW, the Mullard EL34 datasheet has cathode biased operating points at 375 volts using a 3K5 load.

[shooter]

One of my best was a EL34 cathode biased ~~380plate vdc.  I used OT Iron for a Marshall 100 makes magic juice 

[SnickSound]

IMO, the Hammond 270HX power transformer that you have selected is a good fit for this project. For the output transformer, the Hammond 1760J looks like a viable solution; thirty five watt rating with 4k primary windings and 4, 8, 16 ohm speaker taps. The 4K load would be a good fit (IMO) for cathode biased EL34s. A 100% mismatch would provide an ideal (IMO) 8k load for 6V6s. FWIW, the Mullard EL34 datasheet has cathode biased operating points at 375 volts using a 3K5 load.

Indeed at the Mullard datasheet. Speaking of which, Hammond has a set of transformer for a Vox TB35C2 which uses 4 6V6 for 35W: https://www.hammfg.com/electronics/transformers/guitar/tb35c2
PT would be good for 400Vish with diode rectification, with 250mA current handing. The 6A filament line can definitely handle the 5 12AX7 and 2 EL34s I'll be using. It can't do tube rectification though but it has an odd 2nd 6.3VAC line at 250mA... I was going to use the 5V line for my relays, so that is just as good.

The OT is 3K8 load at 35W, close enough. Only downside is no 4 ohm tap, so if I want to run a pair of 6V6s I have to use a 16ohm cabinet to double the impedance.

But I'm tempted to go ahead and put in 4 octal sockets so I can do dual EL34s or quad 6V6 depending on how I feel. I'd probably go with more like a 360R per side for the cathode bias since we're looking at 400V. Then you can put the bypass caps on a switch for dual power level (taking the bypass caps out makes a DRASTIC difference when the cathode resistor is not shared, but it changes the sound a lot since it's basically like adding tons of negative feedback).