6550 OT Primary Z Question

[bepone]

Ug2 is critical first to determing OT load line, and then is also Ua a factor..

before mentioned https://www.vtadiy.com/loadline-calcula ... calculator

is a nice loadline calc. it is possible to play wth Ug2 voltage and Rload, and see how is better to find nice working point with lower Ug2

[martin manning]

I was checking the 6550 spec sheet on this thread and for class AB, two matched tubes and fixed bias the recommended primary at 450V is 4K. I am thinking of building an amp with 525V (plates) so a bit higher. But for a quad of tubes the primary at 450V is half = 2K. I am wondering if the primary should be something like 1.5K or 3.4K for the quad 6550 at 525V

I'm looking at 6550A GE data sheet, which shows for two tubes at 450V Va a 3k5 Raa, and 310V Vg2. At 525V Va, a 2k load looks good for four tubes with Vg2 at 330, and that would make ~200W. I would not go any lower than 1650 ohms, with 370V Vg2.

[Bombacaototal]

I was checking the 6550 spec sheet on this thread and for class AB, two matched tubes and fixed bias the recommended primary at 450V is 4K. I am thinking of building an amp with 525V (plates) so a bit higher. But for a quad of tubes the primary at 450V is half = 2K. I am wondering if the primary should be something like 1.5K or 3.4K for the quad 6550 at 525V

I'm looking at 6550A GE data sheet, which shows for two tubes at 450V Va a 3k5 Raa, and 310V Vg2. At 525V Va, a 2k load looks good for four tubes with Vg2 at 330, and that would make ~200W. I would not go any lower than 1650 ohms, with 370V Vg2.

Many thanks Martin. I was looking at the Music Man OT as used on the Alligator and primary is 2.8K for a quad 6550. that PT is probably putting more than 600V on plates.
I'm still wondering if 3.4K for a quad at around 500V to 525V is far from good. Maybe will put 125W?
I'm talking ultralinear but this has been assumed I guess

I think for a quad 1.7K might be the optimal and 2k is probably plenty as you mentioned.

[martin manning]

I'm talking ultralinear but this has been assumed I guess

No, I was looking at standard pentode operation, which is why I have been quoting screen voltages. On the GE 6550A data sheet there is a 450V Va UL example that specifies 4k for two tubes, as opposed to 3k5 for pentode operation at the same Va. However, it looks like at 525V Va, a 2k primary will work well with four tubes at 42% distributed load. There seems to be a spot there for 6550's where you could run either mode with the same output transformer. An advantage with UL is that you don't need to come up with a separate screen supply voltage (the 330V I mentioned above).

[martin manning]

I was looking at the Music Man OT as used on the Alligator and primary is 2.8K for a quad 6550. that PT is probably putting more than 600V on plates.

If you have 600V Va and a stacked reservoir, using either a voltage doubler or a FWB on a center tapped HV secondary, you have a half-Va screen supply available. With that scenario you'd have a good match at 2k8 Zpri for four 6550's, and get close to 200W.

I'm still wondering if 3.4K for a quad at around 500V to 525V is far from good. Maybe will put 125W?
I'm talking ultralinear but this has been assumed I guess...

That doesn't look promising for UL with typical distributed load fraction of 42%; the primary impedance is too high. It's not bad for pentode operation with Vg2 at half of Va, but it's an inefficient use of four 6550's.

[Bombacaototal]

Thanks again for the input Martin.

It's my first time looking at non Fender style transformers, and the 6550 has always been on my list.

One other option I Iooked at is the Sunn OT which has 2.5K primary. I think it's wired Ultralinear and probably putting 500V+ on plates.

I never tried UL wiring and was curious to try but maybe should stick to triode. I tried a Pentode from the Fender Custom Shop dual professional once.

[didit]

Hello --

If the Sunn amp in question is from early 1970s it would be an A451 as mentioned a few years back in the thread, it has 2k2 primary along with UL wiring. The power section in Sunn's schematic for the 2000s is an example. Literally copy/paste of Dynaco's quad pentodes UL design target 200-250W only limited at the low end, eg. ~30Hz.

Best .. Ian

[Bombacaototal]

Hello --

If the Sunn amp in question is from early 1970s it would be an A451 as mentioned a few years back in the thread, it has 2k2 primary along with UL wiring. The power section in Sunn's schematic for the 2000s is an example. Literally copy/paste of Dynaco's quad pentodes UL design target 200-250W only limited at the low end, eg. ~30Hz.

Best .. Ian

Thanks for the input Ian. I think the early ones had the squared Dynaco OTs (top pic). I was thinking more of the Schumacher one (bottom pic). Which of them had the 2.2K primary? I was talking to mercury magnetics who offer a Sunn2000 replacement OT and theirs is 2.5K

[Bombacaototal]

This Peavy OT at 2.1k is looking like a good candidate

https://www.tubeampdoctor.com/en/output ... -6505-6534

[didit]

Hello --

If the Sunn amp in question is from early 1970s it would be an A451 as mentioned a few years back in the thread, it has 2k2 primary along with UL wiring. The power section in Sunn's schematic for the 2000s is an example. Literally copy/paste of Dynaco's quad pentodes UL design target 200-250W only limited at the low end, eg. ~30Hz.

Best .. Ian

Thanks for the input Ian. I think the early ones had the squared Dynaco OTs (top pic). I was thinking more of the Schumacher one (bottom pic). Which of them had the 2.2K primary? I was talking to mercury magnetics who offer a Sunn2000 replacement OT and theirs is 2.5K

This all makes sense. Noting rectification evolved to silicon on the later model, which also makes sense. Schumacher doubtless made Sunn a UL transformer and a modest increase in impedance insignificant in overall design target. Nowhere I could see showed it as 2k5 primary but was brief on my failed research, and it makes sense. And MM will make you a solid if expensive "replacement".

Best .. Ian

[didit]

This Peavy OT at 2.1k is looking like a good candidate

https://www.tubeampdoctor.com/en/output ... -6505-6534

Unless it's hidden this is not a UL output transformer, but perhaps that isn't your goal.

Best .. Ian

[martin manning]

This Peavy OT at 2.1k is looking like a good candidate
https://www.tubeampdoctor.com/en/output ... -6505-6534

As Ian says, this is a pentode-mode OT. It is rated for 120W. 4x 6550 at 525V will deliver ~200W at 2.1k in either UL or pentode mode. 5150 transformers are designed to work with 4x 6L6GC. You should be looking for a set of transformers with the correct B+, OT primary impedance, and power rating for 4x 6550. If you don't want a 200W amp, it doesn't make a lot of sense to use four of them. A pair can produce 100W.

For the screen supply in pentode mode, this thread describes a MOSFET screen supply and a successful experiment by bcmatt: https://ampgarage.com/forum/viewtopic.p ... 78#p440778
dreric's SSS, which uses Sunn Sorado OT and a fender Twin PT running a pair of 6550's is here: https://ampgarage.com/forum/viewtopic.p ... 37#p378537 Exciting read, with drama and plot twists!

[didit]

Hello --

Will briefly add that in my scanning for a451 info recall seeing that Heyboer has, at least in the past, built/sold reproductions though best understanding without cosmetics of the Dynaco "can". Perhaps they would supply a more affordable option that's fulfilling.

Best .. Ian

[Colossal]

I used a Heyboer built A-451 for a 4x6550 bass amp running in UL. Sounded fantastic. The OT weighs 12kg!

[didit]

I used a Heyboer built A-451 for a 4x6550 bass amp running in UL. Sounded fantastic. The OT weighs 12kg!

QED ..