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Old 8th September 2007, 11:56 AM   #1
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Default big valve traffo for guitar amp?

Hi,

I have a potted power transformer with 450-0-450@230mA and 580-0-580@175mA secondaries.

I want to use it for a guitar amplifier and havent built a powerful valve amp before.
Any suggestions on what tube configuration in the output stage would lap up these transformer specifications?

I know its good for 150W continuous which is nice , i dont have an output transformer yet so i have a blank canvas!

Very interested to hear some opinions of what will be a good match! maybe i could find an excuse to incorporate my MV rectifiers (naaa thats silly).
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Old 9th September 2007, 09:49 AM   #2
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Bit of research and i think 4x EL34's in // PP looks do-able, possibly a pair of 807's.
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Old 9th September 2007, 12:36 PM   #3
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If you are building a guitar amp that will ever be sold you must think about the eventual retubing that will happen. The average guitar player will take the amp to a guitar amp tech or a music store. The selection of available tubes from these two sources is very small. For the voltage levels you speak of there is the EL34, the KT88, and the 6550.

If you are building the amp for your own use you can use whatever you want. The quick thought would be a pair of 813's but the filaments get rather fragile after a lot of use. The more conventional design would be to use sweep tubes (TV line output). I built a few guitar amplifiers that used a pair of 6LW6 sweep tubes to make 150 watts from a 700 volt supply about 10 years ago.

OPT ? I used some old military surplus power transformers. The 2200 VCT secondary became the primary, and the 115 volt primary became the secondary. Not exactly HiFi but it made for an extremely loud guitar amp. At least one of these is still alive and working. I retubed it for the first time about a year ago.
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Old 9th September 2007, 01:46 PM   #4
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Tubelab - I love your style!

As for the transformer - if you haven't built a big tube amp - please be careful. You are talking about 1160VAC (or 900 if you use the inner taps) either of which can kill you easy enough. Please do your homework.

Aside from that it should be good to go. I second the using standard tubes - ones in current production - in 20 years a large number of NOS tubes available now - will be gone.

As was said, a quad of el34, 6550 or KT88 will give you lots of power - and meet the easy to find and replace need.
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Old 9th September 2007, 01:56 PM   #5
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This amp will be for personal use , i am intrigued by this 6LW6 tube, i cant find any data on google about these tubes?!
I can squeeze the output transfomer down to 738VDC from my 580-0-580@175mA tap (it has -10% taps), which is ballpark for 150W from these tubes.. Hmmm.

If they are cheaper than EL34/6550's then this is quite attractive.

As for OPT, i would try and go surplus too, buying new ones is too much for me.. but this is only in the idea stage at the moment so i havent given OPT much thought. Can you get the frequency response needed for a guitar from a power fransformer in reverse?
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Old 9th September 2007, 02:02 PM   #6
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Thanks for the warning, i know how dangerous this will be. Hopefully i will always be respectful when its on test bench and ill stay alive
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Old 9th September 2007, 06:21 PM   #7
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Quote:
i am intrigued by this 6LW6 tube, i cant find any data on google about these tubes?!
The 6LW6 was from the end of the vacuum tube TV era. They are the biggest sweep tubes that I have run across. Several varieties were produced, and some have huge heat radiating fins welded to the plates. These can dissipate 60 to 80 watts without showing color. The 6LW6 is getting hard to find in the US and probably even harder to find elsewhere. 10 years ago they could be bought on Ebay for $5 each. I bought all I could get. Now they fetch $10 (rarely) to $30 each.

See this thread for data and alternatives:

Where to get tube spec sheets, pinouts

Search the tube forums for 6LW6 to find other interesting info.
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Old 10th September 2007, 08:14 PM   #8
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Thanks Tubelab, your information is much appreciated!
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Old 7th October 2007, 04:43 PM   #9
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Hi,

Have obtained four 807 tubes and have sketched out a brief circuit topology for this guitar amp.
2x anode follower gain stages, one LTP splitter driving MOSFET source followers driving 2x 807 tubes in AB2 push pull. Im aiming to put the HT at 500v for about 75W.

Tubelab, you mentioned using power transformers as output transformers, i think i can get some huge filament transformers at LEAST 15A 6.3V. Can these be used with speaker connected accross the heater taps provided they can be wired for 115V to 240V primary?
How does it sound using a PT for an OPT in a guitar amp?
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Old 7th October 2007, 09:04 PM   #10
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Just an FYI, but have you seen the STC 807 datasheet? It's 5.5M and has a 75W AB2 design near the end.
http://www.retrovox.com.au/STC807.pdf
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