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Old 19th January 2004, 07:15 PM   #1
zobsky is offline zobsky  India
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Default next project: rebuild using fisher 500c trannies

Hi,
for my next project, ... i want to build a higher power push pull amp.

I have snagged off ebay,a scavenged power/ OPT pair for the fisher 800c (electrically almost idential to the popular 500c reciever). this piece had a push pull 7591 amp. therefore, my choice of amp is dependant on this set.

i hope the trannies etc are not shorted etc . i should be getting the items next week.

i wish to

the transformers were originally meant to be used with 7591 output tubes.

q. these tubes have recently been re-introduced into the market. how are they, compared to the hard to find NOS tubes (i read opinions on the internet saying the replacements are basically 6L6 tubes)


q. apart from 7591, are there any other output tubes i could use in this circuit (sorry, i don't have the tranny numbers with me although a 500c schematic can be found at http://www.fortunecity.com/tinpan/beegee/290/page4.html ).



thanks
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Old 19th January 2004, 09:01 PM   #2
mcs is offline mcs  Denmark
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Default Re: next project: rebuild using fisher 500c trannies

Quote:
Originally posted by zobsky
I have snagged off ebay,a scavenged power/ OPT pair for the fisher 800c (electrically almost idential to the popular 500c reciever). this piece had a push pull 7591 amp. therefore, my choice of amp is dependant on this set.

q. these tubes have recently been re-introduced into the market. how are they, compared to the hard to find NOS tubes (i read opinions on the internet saying the replacements are basically 6L6 tubes)
No, the Sovtek 7591 tubes are real 7591 tubes. The 7591XYZ tubes they used to make were re-based 6L6s.

Quote:
q. apart from 7591, are there any other output tubes i could use in this circuit
If you mean with these transformers then yes - 6L6, EL34, 6B4G and many others. If you're talking about the exact same circuit as the 500C, you can only use the 7591, 7868 or the 6GM5. You can change the circuit to suit other tubes though.

Best regards,

Mikkel C. Simonsen
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Old 19th January 2004, 09:12 PM   #3
zobsky is offline zobsky  India
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Default Re: Re: next project: rebuild using fisher 500c trannies

Quote:
Originally posted by mcs




If you mean with these transformers then yes - 6L6, EL34, 6B4G and many others. If you're talking about the exact same circuit as the 500C, you can only use the 7591, 7868 or the 6GM5. You can change the circuit to suit other tubes though.

Best regards,

Mikkel C. Simonsen
thanks for the info, ...
At this point, the only component that is fixed are the output tranny combination, ..

i'd like to reuse the same power transformer but if i can build a better amp using an (output compatible) tube which needs a different voltage, i can get another power tranny with not too much trouble.


opinions, ... recommendations, ... attributes of different tube combinations, circuit topologies .... <fill in here>

speaker efficiency is not an issue here, since i have a bunch of various types at home,.. more so is the best possible sound i can get using tubes compatible with the available output transfomers (at a real-world cost).

thanks very much, .. keep the replies coming.
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Old 20th January 2004, 06:19 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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If you want to use nice old NOS tubes, try an 807. They are available DIRT CHEAP and have a similar plate dissipation and specs to a 6L6GC. They are also quite beautiful with their ST shape and topcap. They sound great in UL or pentode (not so good in triode IMHO.)

I recently bought a quad of NOS 807s for US$20, another quad was given to me! Try buying a NOS quad of EL34s or 6L6GC for that sort of price!

I used the schematic found at http://hollowstate.netfirms.com/DIY_files/proto.htm
except with an Em81 magic eye and cathode biased output tubes in UL... Also only about 250V on the 6SN7s. Sounds great!
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Old 26th January 2004, 03:53 AM   #5
zobsky is offline zobsky  India
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Hi

the trannies should be here by the end of next week.
narrowing down the design goals a bit

1. choice of tubes:7591 or 807 , .. the 807s are about 3-4 times cheaper than 7591 tubes. they certainly look cooler, ... i see a lot of varying opinions on 807 tube amps on the www, .. but could this be more due to circuit implementation rather than the tube itself?

any other good tubes i could use?

2. circuit choice, .. i guess i could try almost any of the stuff in the push pull section of the fi-primer. ...but can anyone suggest an even better tried & proven circuit?

thanks
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Old 26th January 2004, 04:00 AM   #6
mcs is offline mcs  Denmark
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Quote:
Originally posted by zobsky
Hi

1. choice of tubes:7591 or 807 , .. the 807s are about 3-4 times cheaper than 7591 tubes. they certainly look cooler, ... i see a lot of varying opinions on 807 tube amps on the www, .. but could this be more due to circuit implementation rather than the tube itself?
The 807 and 7591 are VERY different tubes, so don't expect them to sound the same

But both tubes are good, so use whatever you want...

Quote:
2. circuit choice, .. i guess i could try almost any of the stuff in the push pull section of the fi-primer. ...but can anyone suggest an even better tried & proven circuit?
How about a Williamosn? Even better - who knows? But certainly tried and proven - have a look here: http://stiftsbogtrykkeriet.dk/~mcs/ (at the bottom of the page).

Best regards,

Mikkel C. Simonsen
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Old 26th January 2004, 05:27 AM   #7
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The Fisher OPT's are a 6k6 design of reasonable but not great quality. I own an 800B and have "borrowed" the OPTs to try in other projects. The Fisher uses a voltage doubler in the power supply so the trans has quite a low voltage. The schematic is not to hand, but it's about 150-170V. Personally I'd ditch it and use a FW with 12AX4GT's/6CJ3's, but that's just my preference.

Depending on how you want to run the output tubes (triode or pentode) and the amount of power you want from them, there are a ton of options from 2A3/6B4G to 7591/EL34 pentode amongst others.

The 800B's are supposedly rated for about 37W/ch, and the max continuous current recommended was 60mA-ish for the OPT's (thinking class A triode). There are no UL taps on the primary, but I've never heard a really great UL amp yet.

Shifty's 807 idea is good. they're great tubes and cheap, but running them high voltage (~400) in triode sounds crap but gives max power. 300V/70mA per tube works better sonically but is low power, and in pentode, the STC datasheet from the Retrovox site gives excellent starting points, esp if you study the distortion curves.

The new 7591 is a good tube. Mine haven't given any problems in service and sound fine in the original circuit. I've not tried them much elsewhere, or trioded in the Fisher, as the Fisher is a nice lush sounding but only modest resolution design.

If you don't need a lot of power, I'd go with the 2A3 stigla/RPower PP design. Do a search here to find details.
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Old 31st January 2004, 06:43 PM   #8
zobsky is offline zobsky  India
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update, i've beenreading up a bit.

1. i recieved the power / output transformers this week. Brett's suggestion to chuck the power tranny is wise

2. the next question is which circuit to use. Mikkel suggests williamson / mod williamson. my vintage 6v6 pilot amps are williamson, but i can't bear to modify them. i guess i could build a williamson but i was looking for something more exotic, ... the idea of using interstage transformer as a phase splitter in the ralph PP appeals to me. in this regard:
a. how expensive are lundahl ITs, .. couldn't get an idea off their web page but i'm estimating $150 or so per pair. i have seen references to ha
b. are their any cheaper alternatives which perform as well? I have seen http://diyparadise.com/pt156.html but would like to hear from someone who may have tried this.
c. would using this kind of topology with my fisher transformers be like putting skinny tires on a sports car ie. bottleneck being the fishers?
d. parafeed vs. regular IT, .. can someone give me the lowdown. From my readings, parafeed makes things a bit more economical by ensuring that the IT doesn't have to deal with DC (courtesy of the DC blocking cap) but in doing so,it complicates matters in that we introduces an additional component in the signal path, as opposed to conventional IT coupling
e. kind of a naive question, ..but does conventional IT coupling have the same benefits as plate choke loading, as the feed from the plate passes through the primary of the IT?

2. still not sure which path to follow as far as tubes to use.
a. 7591 tubes are about $75 a quad (new sovtek) and will give me more power
b. Brett, what is "FW" as in "Personally I'd ditch it and use a FW with ...."
c. still considering the 807 vs. the 2a3/6b4g. something like $50 for a quad vs. $150 for a quad for the 2a3. as far as tube flavours, my listening experience in tubes so far has been limited to 6bq5 (SE, pentode / triode), 6v6 (push pull), DHT 2a3 (SE - bottlehead paramor). Brett, you suggest 807 in pentode, as opposed to tride at lower voltages. what is the opinion based on (distortion curves or just the sound signature .
d. the fi primer also suggests a DC coupled 6ck4 indirectly heated triode (http://tinpan.fortunecity.com/saints/668/primer/pp.html , http://tinpan.fortunecity.com/saints...er/6ck4-pp.gif). any opinions about this, .. seems like another elegant design., tubes should run about $45 for a quad.

3. another interesting link i came across describes a rebuild and extensive mods using the same fisher 500c transformers in a IT / parafeed PP arrangement. this project uses type 47 tubes (i have hardly been able to find any sources for this tube on the www) at http://home.pacifier.com/~gpimm/47.htm .) if you notice , he uses an IT as a phase splitter and feeds the output to the driver tubes (the reverse of most schematics). opinions?
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Old 31st January 2004, 07:10 PM   #9
SY is offline SY  United States
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With a 6.6K p-p primary. the EL34 is an obvious choice. EL37 would be even better, but good luck finding those. I'm experimenting at the moment with some 6BG6s that I picked up on the cheap. These versions seem to be pretty much identical to the fabulous 7027As, other than the pin-out.

Want an exotic input circuit? Here's what I'm using at the moment in my Eico HF87 (also with 6.6K o/p trannies). Input transformers are, IMO, a better choice than interstage. You can get fancier with the current source, but I haven't gone in and measured the effect yet; the simple one seems to work and sound fine. The feedback is taken from the 16 and zero ohm secondary taps, with the 4 ohm tap being grounded. As you can tell, I like balance!
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Old 31st January 2004, 10:34 PM   #10
zobsky is offline zobsky  India
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Quote:
Originally posted by SY
With a 6.6K p-p primary. the EL34 is an obvious choice. EL37 would be even better, but good luck finding those. I'm experimenting at the moment with some 6BG6s that I picked up on the cheap. These versions seem to be pretty much identical to the fabulous 7027As, other than the pin-out.
Thanks, .. how do you like the 6BG6 (sound character???), .. are you running them in pentode / triode and at what operating point / voltage?
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