833A (GU-48) Amp

[tentchemise]

Hi pre65,

For the U19, a choke input or cap input with a resistor source (500R - 700R) should work.

Regarding the circuits:

"Bi phase half wave" typically refers to full-wave rectification with 2 valves.
"Single phase full wave" likely means a bridge circuit with 4 valves.
As for filaments, they usually require AC (4v @ 3.3A). Each U19 should have its own filament transformer.

Hope this helps!

[pre65]

Thanks.

Although I do have a few U19 I'm going to stick with 6D22s damper diodes in a hybrid bridge.

As the HT will be similar to the last iteration (950 - 1050v) I'm happy to keep with what seemed to work fine.

Nick's suggestion re making the grid more positive may just be what is needed to get the red spot on the anode which seems to be indicative of a good operating position.

I do have a few 833C which have the carbon anode as well as my trusty pair of GU-48.

[Nick]

which seems to be indicative of a good operating position.

Not sure about that, its fun to thrash them, but to get those sorts of op points working involve getting all the other parts to do their job as well, and that's far from simple. Like gm70's, they can also sound very good at much lower dissipation, just needs the right speakers. Its very easy for any improvement that high power op points may provide to be swamped by the negative effect that badly done (or just mediocre) power supply and or driver.

And of course good quality iron that will handle the higher power is very hard to come by. Don't underestimate the contribution made by those Tribute transformers that Paul brought to that amp at Owston. Those TX's are in effect unique AFAIK and unlikely to be repeated. AE might get you 90% there, maybe.

[pre65]

OK.

I had my 833a amp at Owston at the same time you and Paul displayed your joint version.

I think my version stood up quite well, my OPT are Hammond 1642SE, not the best, but not cheap crap either.

Can you remember if my version sounded "inferior" on the day ? Be as honest as you like.

[Nick]

Can you remember if my version sounded "inferior" on the day ? Be as honest as you like.

Well, to be honest (not sure you can be slightly honest), but I can't remember. In general when I hear your amps they have been let down (IMHO) by either the speakers and or music played, so I don't have any real info to give about the amps.

not the best, but not cheap crap either.

In the company of the Tributes, they are effectively cheap crap though unfortunately.

[Paul Barker]

I agree and the acf2 would make it an unfair comparrison.

At the moment Im surviving, building an amp into A2 is not high enough on Maslow’s hierachy of needs in a snapshot of my life as it is now. I may never get back to it as far more urgent basic needs are neglected.

[andrew Ivimey]

Oh ffs, I was just finishing a post and eBay interrupted about a bid for already overpriced Fender guitar pickups...

When I played with 833s or Soviet equivalents I was always worried that when voltage went down current went up and 350ma and climbing just felt like too much to go on with. Your 'crap' transformers are Hammonds and I'm would ask, are they safe at lower HT because the current is getting cooking.

Fidelity and glory, like red hot anodes come later, perhaps. Bias that grid baby! ( I loved heating the anodes of 6s45s or whatever those glorious 1 watt spud valves were.) If the Trafos are 'safe' then Onwards, down the road you go.

Having got a pair of gu whatevers going. I really didnt bother with any kind of smoothing/ filters on the heaters- they were straight computer psus - hoorah for swps - rugged! I left the shed and went for a walk leaving a dvd player on repeat with Ritchie Blackmore smoke on the watering and the rest.( I felt that in a small way the irony would not be lost on all the fire crew and boys in blue ( yeah alright it would, wouldn't it) I returned a couple of hours later to hear marvelous Spl; dBs a plenty, shed was gloriously warm and 'amp' as such was very hot but really, a bit much!? Safe? Nah.

I sold my gu48s and rested 'comfortably ' in the gutsy welly of GM70. Mark sold me a lovely mains transformer that improved matters in a reliable if also geographical boat anchor wattish kind of way. 6d22 rectifiers too but the bill has to be paid!

These days I have a box of svetlana 6550s and a quad of kt88s from China waiting but like Paul and Mr Maslow I have reasons for not getting on.

Oh my!

[pre65]

I seem to remember once we were playing with an idea to drive the 833 with an EL34 or something? But didn't work out...

You are right Mike. You did a lot of work to help me in the amps early days and I thank you again.

I've been re reading the thread from post 1 and I'd forgotten how many things were tried before I got the interstages that are used now.

[pre65]

Can you remember if my version sounded "inferior" on the day ? Be as honest as you like.

not the best, but not cheap crap either.

In the company of the Tributes, they are effectively cheap crap though unfortunately.

If Paul reads this, Paul, if you ever decide to sell the Tributes can I have first refusal ?

[Paul Barker]

That’s a tough call. I planned a 212 amp for them, for which Pieter was tasked. Less likely I’ll build it but not ready to give up, just dormant addressing survival.

[pre65]

You are right Mike. You did a lot of work to help me in the amps early days and I thank you again.

Of course, it would be wrong not to mention Nick, whose guiding hand and expert knowledge has helped all that have been in need of advice, and also Paul.

Nor to forget those who have been enthusiastic about my progress and chipped in with helpful and amusing comments from time to time.