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GM70 an 813 strapped in SE-UL - circuits?

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Does anyone have a tested circuit for a GM70 SE, or PSE; an 813 strapped in SE-UL?

My main speakers (Apogee hybrids) are only 84 dB, so 100 watts would be preferable. Though it might add too much to an already substantial project . .

I could go KT88 PP UL/ pentode mode, but would probably prefer SE.

How many watts would an 813 strapped in SE-UL produce?

Any suggestions on sources for OPTs and 240 volt power transformers?

Thanks
 
suitability for ultralinear

Thanks Arne,

Parallel UL SE could be the way to go. :)

It doesn’t have to be an 813 -

I have read that eg of the 6V6, 6L6, 5881, EL34, 6CA7, KT66, etc clan, the KT88 is best for running in ultralinear mode.

How can the suitability of tubes for ultralinear be determined?

Thanks
 
I sympathise - I have Apogee Caliper Sigs.

First thing is, depends on room size. I drive my Apogees with a PPP 6S4 amp - can't be more than 10 watts. A SE300b wouldn't drive them at all. A PP KT88 amp drives them fine - all this in smallish rooms. So if your room is not too big, I'd say go with PP300b or PSE 300b.

I'm thinking 813 myself - I've bought a stock. Personally I build all DHT - not interested in the directly heated valves unless I have to use them for some reason. I prefer the sound of DHTs.

I'm thinking push pull 813 myself. I like SE but it's more important to me to be using DHTs. I don't want to go high voltage so I'm thinking 450v. But with 813 you can go higher.

Andy
 
hey-Hey!!!,
I have a similar valve to 813 running U-L at ~550V, the 4E27. Nearly pin-compatible with the 813, but at 5V instead of 10V on its cathode. I have special outputs made with multiple taps, and taken from the Peerless S-265-Q coil geometry.

I will swap to 813 as soon as my leisurely collection of a quad is complete. The filament supply is easily converted too...:) Bias will be similar. The U-L percentage is critical for the load matching; it changes the operating elvelope of the plate curves.
cheers,
Douglas
 
Andy

Apogee Caliper Sigs with 10 watts? Even in a modest room, that’s amazing!
Apogee guru Graz suggests that Apogees need plenty of watts to sing to their best . .


I have only Centaur Minors, and 50-150 watts is recommended. The room is 5.3 * 6.7 * 3.5 m (c 17’ * 21’ * 12’) so I think I need 50+.

Douglas

> The U-L percentage is critical for the load matching; it changes the operating envelope of the plate curves.

Maybe its best to get OPTs with taps at say 40%, 50% and 60%, to allow thane amp to be better optimised.

Shane

I’ve been to the GM70 Yahoo group a little, but hadn’t seen much there. I should go look more.

Has Tubelab said that his SE board could be used for GM70's? :cool:

What power and output iron will you use?? $? :eek:

Cheers
 
Hows' this...An 813 strapped in UL.
_________________________________________Rick.............
 

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otto88 said:
Andy


Has Tubelab said that his SE board could be used for GM70's? :cool:

What power and output iron will you use?? $? :eek:

Cheers

To my knowledge he has never tried it or mentioned it but has used it to drive 845's, 211's and and 833A so I do not think there will be a problem.

I have some Chinese 10K 50W SE irons that should take the punishment.


otto88 said:
Shane

How did you determine the (initial) component values for the SE board with GM70's?


I am starting with the component values that tubelab used for his 845 and experimenting from there.

I have most the parts for the drive section now and a set of 45's on the way. I now searching for power supply parts
 
Douglas

> Go lower than 40% too; on this I must say, " you may be pleasantly surprised".

What % do you use, and what output do you get?

Shane

I have most the parts for the drive section now and a set of 45's on the way

Will 45's be sufficient to drive GM70s – I would have thought PowerDrive might be better?


Josh

What might an 813 parafeed output?

Thanks
 
Based on George's comments on his site about the 845 amp with power drive, I'd think the 45 would drive the GM70 or the 813 just fine. Both have higher gain than the 845, so they require a bit less swing than the 845, which was the limiting factor George mentioned.

My idea to use the 813 in parafeed was not based on maximum power output. I was thinking of probably triode strapping the 813 and running parafeed. The parafeed idea is mostly to sidestep one of the issues in building higher powered SE amps, the OPT. With parafeed one can use an ungapped transformer and gain wider bandwidth in a higher powered transformer without as much trouble and expense.

I imagine it may be possible do a UL parafeed amp with a plate choke that has a tapped lead. I don't know who makes one.
 
Thank Gluca,

I've seen your posts on Mike's forum. Your designs are part of my inspiration. I was thinking Magnequest when it comes time to embark.

P.S. I am building a variation on your 801a preamp. Not too much variation, but I won't be using the exact PSU you did. Not sure I want to deal with mercury rectifiers, so damper diodes will be my choice.
 
GM70's

I have messing with GM70's in push-pull class A/AB. I was looking for a big single ended amp as well but I thought the GM70's in push pull sounded the best without having to spend thousands on SE OT's. They sound very good and pack enough punch to listen to my Magnepan MMG's at decent volume. (84 db efficiency).

Basic configuration:

CCS-loaded 6JC6A -> triode strapped russian 5881 (6P???)->fender 300ps interstage transformer-> GM70's -> hammond 5K 100w push-pull OT

The GM70 need at least 750 volts with some decent amount idle current to hit that sweet spot. I have been switching out the 5881 with some VT-52's and VT-25's but the the triode strapped 5881 give the flatest and widest frequency response.

Scott
 
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