Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on August 08, 2014, 04:09:17 pm
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Hi guys, Couldn't pass this one up when a local guy had it up for sale.
My thoughts were to recap it and do the usual SAFETY things, a tart up of the cabinets (comes with a speaker cab)
It has had some MODS done and I have removed some of these for safety reasons and they were not needed.
I was able to get a good tracing from the other amp, so this will be my guide for the circuit restore.
I was told by the seller that this amp was used a gigging BASS amp for many years.
With help from the boys here in OZ it was determined that the original PT got fried as well and the new PT install and MODS were done early in the life of the amp (components of the day and A&R power transformer)
As you can see the B+ filtering was beefed up and a DUAL fixed bias installed and EL34 power tubes.
Again, I am faced with how to restore it to a similar circuit to the original and NOT put components under STRESS.
I posted this http://el34world.com/Forum/index.php?topic=17413.0 (http://el34world.com/Forum/index.php?topic=17413.0) in an effort to avoid replacing the 6CA4 with a GZ34 as I did in the other amp. The new PT has extra taps 6.3v and 5v for some options.
A SS rect was used as the PT only has 285-0-285 secondary tap and this replaced the rect tube.
I was a bit concerned that having a Fixed bias may invite someone to bias it a bit hotter and overload the OT that is rated at 25w and may not be a good match for the EL34s.
Please add some thoughts.
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More.........
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SCHEMATICS
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I was a bit concerned that having a Fixed bias may invite someone to bias it a bit hotter and overload the OT that is rated at 25w and may not be a good match for the EL34s.
If someone future owner wants to open it up, take a hot soldering iron and tinker with the bias, fixed vs. cathode bias doesn't really matter.
I see 5-6 diodes and a full wave bridge rectifier, so why worry about a tube rectifier?
how does it sound AS-IS?
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6CA4 is the same as EZ81. That just seems like a bad match for a pair of EL34s. I don't think I've ever seen that match before.
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The E-Caps are just about to pop so I was wanting to order some new ones but I did not want to replace them as is but return it to stock.
Caues the PT is a little low on its output it would be better with s SS rect which I think I will mount the diodes separate to the socket so on one can put a tube in.
I will remove the choke and put in a 3k resistor as per the original and screen resistors on the power tubes.
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Sluckey, I don't think the EL34 were stock but whoever modded this was happy enough to put them in but I'm not sure that they are a good match for the OT, as stock would have been 6L6s but if the ELs are ok for the OT (two primary taps 8k and 6.6k) and secondary 3.7, 8 and 15ohm.
As this is already setup with a fixed bias, what would be better suited for this amp, FIXED or CATHODE????
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Big tubes, ~~400V B+, 6.6K OT..... I would cathode-bias this. It won't be much over 20W-25W output in any case. Cathode-bias is simple and safe.
If you wanted BIG power, you'd re-load at 4K or 3.5K. Then you would run fix-bias for cool idle with BIG peak output. However that PT may or may not hold-up under the increased load. And high-Watt tube amps are not that hard to find, even new. I wouldn't really want to hot-up a Classic just for a few Watts more output.
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Thanks PRR, With the SS rect I should get around 400v and the el34s ok with 6.6k. Thanks
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> 400v and the el34s ok with 6.6k
I down-rated a Too-Darn-LOUD Ampeg VT-40 with 6.6K output from 590V to 400V, stuffed with EL34s, idled 100mA both cathodes together, and it was pretty sweet. And IMHO far more reliable.
I remember 250 ohm common cathode resistor and 3K common screen resistor. That screen R is too large for Max Power but does make it very forgiving of abuse.
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Oooooooooooo, Large screen resistor, what does that mean.
The original or maybe this one did not have a screen resistor/s, but is wired with a single resistor and at the moment is 560 ohm