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In Reply to: Re: Gabe's mag-1515 PP EL84 amp? posted by andy evans on August 26, 2002 at 13:27:07:
How do you think this amp would compare to the Decware Zen?
The Zen has a great reputation but I want more power.Decware Zen amps use SV83's but can also use EL84's.
Does that mean that SV83's could be used with Gabe's amp to have less euphonics (but also less power)?
Follow Ups:
A SV83 is essentially an EL84 with a lower screen voltage rating. So you can use '84s in a sv83 amp, but not the other way around, unless you make modifications to lower the screen voltage. (presuming that a 15w el84 amp would likely have the screen at around 300V)Both tubes are thought to be good sounding. There are several NOS el84s available to try and the sv83s are good sounding and also dirt cheap.
The original designation for the SV83 is EL84N. As long as the SV83's screen grid is protected from overvoltage, it can be used in place of the 6BQ5 without problem. The SV83's pinout is slightly different than that of the 6BQ5. However, an amp wired for SV83s can use 6BQ5s without any changes.PP designs cancel even order harmonic distortion. That cancellation plus NFB will yield a very different voice from that of the triode wired, single ended, ZEN SE84.
Eli D.
For those starting from scratch, it's fairly simple to add another pair of EL84s to double the output power, giving a very respectable 20-25 watts. Power supply requirements for a monoblok are the same as a stereo EL84 amp so plenty of iron about - 200mA chokes and Mains transformers. EL84s are easy to drive, so there should be few problems with a conventional Mullard circuit. In theory the output transformer should be 4K primary, but in practice you're more likely to find a 5K such as Lundahl or the Hammond 1645 (I've just ordered a pair of these for exactly this purpose). Sowter do 4K OPTs at about £100 each, and of course many winders will do these to order, such as Majestic. Nothing to stop you adding another pair of EL84s again (six per channel) which is what the Bill Beard amps did - they sounded very nice too.
Is that it's a pain in the but to get all those tubes running the same. Make sure the bias can be adjusted for each output tube or you'll go nuts, even so you still might stress yourself out some.
Interesting post by Scholl - but why should this be such a huge issue? As long as the draw is somewhere between 10 and 12v on the cathode, the valve should be within operating conditions. Obviously matching is going to help, but surely it's not a fine art? I spoke to the ex-Beard designer (Chris Found) about this and he said that while in later models Beard did fit regulators under the EL84s, the main thing was to get a good matching set of valves. He also said that he'd seen the same set of valves last ten years without problems. Can you expand on where the difficulty is - I'm interested in your experience? Andy
There's something of a manufacturer's compromise going on here too. A commercial amp has to be plug and play as far as the customer is concerned. You could set up the bias individually and supply carefully balanced valves, and maybe the client would put the valves in the right sockets as instructed, maybe not. Like wise when it comes to valve replacement, the customer may buy carefully selected and matched valves, maybe not - Either way you want the amp to work. So many amps will not have individual bias adjustments. Using regulators is another way to make biasing customer proof.
I've found that the more pairs of output valves there are paralleled up in an amp, the less matching matters. Individual characteristics of any one valve are 'averaged out' by the others, provided no one valve is grossly off spec.Regards,
Andy
The difference between 10 and 12 volts bias between 4 sensitive tubes like the 6BQ5 can cause enough of a current difference in the output transformer to impact the bass roll off of the amplifier. If a single bias control or cathode bias is used only one bias voltage will presented to the 4 tubes. One tube might produce 40ma at 10 volts another maybe 35 ma another maybe 42ma the forth maybe 38ma. Now if fixed bias where used with an adjustment for each tube and each one adjusted for 40ma it’s not so important if one tube is biased at 10v while another is at 12 etc. dc balance is present in the output transformer for best low frequency performance. These values are used for example only.My last adventure into PPP was with a pair of Frazier 102 monoblocks using 4 6GT5s per channel. I had 18 tubes to work with 10 of them NOS. It took 1 ½ hours to find close sets. The bass response started at 40hz and improved to 24 hz –1db. Who knows what this will look like in two years without revisiting them every several months.
Getting tubes to work well together isn’t a fine art, it just takes time and care, and has to be revisited two- three times a year that’s all. New matched tubes with separated bias controls should make this job easier and faster. Keep in mind if Beard engineers didn’t think it mattered they wouldn’t of added some form of regulation to keep tube bias in check. They would have used a single cathode resistor and saved a few bucks and some time.
Search the Asylums and you’ll find more postings on this subject.
And noone talked about AC balance in this thread. I think I’ll go over to SET Asylum for a while.
Good luck.
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