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Old 18th September 2005, 02:19 AM   #11
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It seems 300B SE builders don’t require muscle?

I guess a schematic of Allen Wright's all-differential, all-Class A DPA-300B might require industrial espionage in Switzerland. (A few years back Dan Marshall on the Joe List was working on an all differential, completely transformer coupled stereo PP 6B4G amplifier, but I don't brlieve published).
Anyone know of any other all-differential, all-Class A audio amps?


Brett

Not boring, and cheap, and a CCS should be good.

But a PP EL 34? It has the power, so it could have the effortless feel.

Still taking in this further lateral thinking.

> Just do a better PSU than Allen's.

What do you suggest?

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Old 18th September 2005, 02:41 AM   #12
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Quote:
Originally posted by rick57
I guess a schematic of Allen Wright's all-differential, all-Class A DPA-300B might require industrial espionage in Switzerland. (A few years back Dan Marshall on the Joe List was working on an all differential, completely transformer coupled stereo PP 6B4G amplifier, but I don't brlieve published).
Anyone know of any other all-differential, all-Class A audio amps?
http://www.rintelen.ch/pdf/DIY/Igor.pdf
Quote:
Not boring, and cheap, and a CCS should be good.

But a PP EL 34? It has the power, so it could have the effortless feel.

Still taking in this further lateral thinking.
Depending upon the voltages you choose to run it at (I like 6550/KT88 hot and low over most EL34 setups) you could set it up using your choice of multi tubes and change it to 300Bs later if you chose with not too much effort.

For O/P stage CCS, go with a Pimm or similar
Quote:
> Just do a better PSU than Allen's.

What do you suggest?
I found large performance gains in this amp with FW 6CJ3, LCLC, all motor run caps and a superreg or simple CCS-> stacked oc3's controlling the front end. I messed around with different powertrans to get the op points I like too. Spend some time in PSUD2. I usually don't bother with dual mono supplies either anymore.

MUCH larger, heavier and more expensive to build than the original though.
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Old 18th September 2005, 02:45 AM   #13
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PS - if you really want power and control and dynamic range that will frighten children and small animals do a version of the AW PP1C using GM70's in the output stage at around 800V/100mA. I had some with 813T and GM70 driving my horns, but I didn't need anywhere near the power.
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Old 18th September 2005, 04:13 AM   #14
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Thanks for the link to Christian Rintelen’s Igor 300B PP.
I’d avoided 300B PPs because of the extra cost. I guess you’d need at least US 1500 to do it properly, about twice my target. May be a project for down the track.

> I like 6550/ KT88 hot and low over most EL34 setups. You could set it up using your choice of multi tubes and change it to 300Bs later if you chose with not too much effort.
For O/P stage CCS, go with a Pimm or similar

Sounding more appealing . .

> I found large performance gains in this amp with FW 6CJ3, LCLC, all motor run caps and a superreg or simple CCS-> stacked oc3's controlling the front end

What were the gains in – bass extension/ dynamics/ overall clarity . . ?

I really like the idea of GM70s. I suspect that would cost even more than a 300 B PP?

Thanks
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Old 18th September 2005, 04:24 AM   #15
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Quote:
Originally posted by rick57
> I found large performance gains in this amp with FW 6CJ3, LCLC, all motor run caps and a superreg or simple CCS-> stacked oc3's controlling the front end

What were the gains in – bass extension/ dynamics/ overall clarity . . ?
From memory (3 years ago) better resolution, especially in the degrees of dynamic response (graduations?), lower noise, less grainy and white, better focus.

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I really like the idea of GM70s. I suspect that would cost even more than a 300 B PP?
I had all the other parts on hand so it only cost some tubes, which aren't that expensive in themselves.
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Old 18th September 2005, 05:29 AM   #16
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Have you compared the sound of your GM70 to a 300B SE (or 6550 or 300B PP)?

> I had all the other parts on hand

Roughly what do you think costs would total, building a GM70 from scratch?

Thanks
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Old 18th September 2005, 08:56 AM   #17
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Quote:
Originally posted by rick57
Have you compared the sound of your GM70 to a 300B SE (or 6550 or 300B PP)?
No, yes, no. Better in all regards. I used the existing PSU and driver from the AW amp and added the PSU for the GM70's. Later I went to big pentode drivers for even more improvement.
Never heard an SE amp that I thought was great.

Quote:
> I had all the other parts on hand

Roughly what do you think costs would total, building a GM70 from scratch?
No idea. I buy surplus iron and find uses for it, and big power trans etc can be very cheap secondhand as few people build high voltage projects.
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Old 18th September 2005, 10:52 AM   #18
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I thought GM70s, if you only need eg 20-30 watts might only need say 500 volts.

Are they like 845s or 211s – need near max volts to sing?

GM70 - I haven't found a datasheet - can you upload one. How many volts is your B+?

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Old 18th September 2005, 02:38 PM   #19
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Quote:
Originally posted by rick57
I thought GM70s, if you only need eg 20-30 watts might only need say 500 volts.
More like 800+

Quote:
Are they like 845s or 211s – need near max volts to sing?
Can't say for 211's, but for 845's, 813T's and GM7o's that's utter c*r*a*p.

Quote:
GM70 - I haven't found a datasheet - can you upload one. How many volts is your B+?
Attached.

All other aspects of the design I had are from memory as it's been packed up for quite a while. Due to severe space limitations in this house, I've been tube ampless for >2 years, except for the MI stuff I build for customers.
Attached Files
File Type: pdf gm70.pdf (19.7 KB, 148 views)
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Old 19th September 2005, 02:49 AM   #20
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My 300B push - pull amp cost me under $300 USD to build. Like Brett I find surplus iron and then build an amp for it. In this case I bought a truckload of push - pull OPT's when the guitar amp company ADA went out of buisness. I used them in the 300Beast, and in many other projects. I have tried several other transformers (including some UTC's and other expensive iron) in this amp and I keep putting back the surplus units. The transformers were 6600 ohm CT units made for 6L6's. Other PP transformers worked well in this amp. The UTC's were too bright for my speakers.

For power transformer I used a 300VA toroid that has 2X 175 volt AC secondary windings. put them in series, then use a SS bridge and a choke input filter with a LARGE cap. The rest of the schematic can be found in Vacuum Tube Valley #12. I used 6CG7's instead of 6SN7's because I have lots of them, and I find most 6SN7's to be microphonic. I used the simple power supply because I had the transformer.

If you decide to build a big SE amp, the secret in power transformers is to find an " industrial control transformer" these are intended to step 480 volts (they come in other voltages) down to 120 or 240 volts. Hook them up backwards and they turn 120 or 240 volts into 480 volts AC. A SS bridge gives 450 to 500 volts with a choke input and over 600 volts with a cap input. I use a voltage doubler made with 5AR4's to get 1100 volts. These transformers can often be found surplus for less than the shipping cost. I paid $20 FOR A 1KVA transformer. These transformers are rated in VA's, get a bigger one than you need, they sound better, due to improved regulation.
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