Discussion in 'Tube Audio' started by fink lever, Dec 26, 2017.
Cool. Using V3 and V4 for G2 is new to me. Looks like you're in good hands with yestertech.
Yes thanks for the link Dandy! He seems a lovely guy and obviously very knowledgeable...He has been amazing really, though i think he is actually enjoying what he's doing! lol
The large opt may have supported 6L6s, the smaller 6V6s. Bias will have to be vetted and assured for whatever goes where.
This is actually fairly common in older amplifiers. Stabilizing G2 of output tubes really improves the performance and sound of an amplifier.As a side benefit, when you turn up the volume, G2 draws more current, which diverts less to the gas stabilizer tubes, hence the glow of the tubes pulsates in time with the music, making a neat looking effect.
I use Goodmans speakers from that era regularly (An Axiom 22, and an Axiom 150 Mk. ii), and they are absolutely wonderful sounding, especially considering their age. It's possible to dismount the magnet and inspect the voice coil on my speakers - maybe yours too. It's great to see this project preserved, please let the forum know how it turns out!
Definitely will do. Still a way to go but it's looking good. And so good to get into the Homebrew side of valve amplifiers, they seem to interest me more being that they are peoples own ideas and creations. One reason for keeping this cabinet...
More info from tech:
2 transformer ( each with a mains voltage adjuster ) + rectifier PSU’s . Main supply has two smoothing chokes, the secondary only one. Plus 2 pentode based push pull amps each using a single triode split-load phase splitter (also known as "concertina phase splitter”);
Note no global feedback s used.
The screen grid supply for all output valves is stabilised via the 2 stab tubes. Good practice !
The secondary supply when enough current is drawn will operate the relay coil and activate the main supply. It does this by closing the centre-tap connection to ground for the high voltage AC secondary.
More to come…
If there are no cathode bypass caps on the output tubes, they have local feedback at that stage. The split load / concertina / cathodyne inverter also has local feedback for the same reason. Overall it makes it relatively linear even without global feedback. With proper circuit design and good parts you can actually get pretty good frequency response from that setup, especially since it doesn't have to cover the full audio spectrum anyway. each amp only has to handle it's portion.
Transformer ratio may be a clue as to what tubes go where. Typical 6L6 is somewhere around a 6.5K plate to plate, 6V6 around 10K, 6F6 10-12K. Voltage and bias resistors also would be a clue. If this was designed using tube mfg's "typical operation" tables on the datasheets you may find that it is literally a textbook example of a circuit.
More info and Update from Tech...
After some more poking around the chassis, I’ve found that the 2 output stages are fed HT from different places. I ‘d missed that the stabilised, lower voltage HT + also feeds the ‘Maroon’ output stage, which I’m hoping you can confirm is the smaller loudspeaker ?
The regulated output of 2 x S130 stabilisers is set at approx 240V so it seems reasonable that this was for the lower power stage, most likely 2 x 6F6 As yet, I can’t confirm what the ‘full’ HT voltage is ( as the caps are unsafe to apply power to ) but probably 300/350 volts at a guess. This suggests a higher power amplifier - enter the pair of 6L6s which were in evidence on the chassis.
The biassing ( 1 common cathode resistor, unbypassed ) is , as expected, different for the 2 amplifiers :
38 ohms + 150 ohms for the ‘maroon’ or 6f6
38 ohms + 200 ohms for the ‘green’ or 6L6 output valves
The 38 ohms i additional bias resistance formed by the balancing preset pots in each amp, assuming it is set dead centre. To match the quiescent current in the 2 output vales, these can be altered in either direction to account of valve tolerances/ageing
None of these values of Rk match exactly those quoted in the WW valve data book for various loadings of 6F6/6L6 so they will be somewhat less than quoted :
The nearest 6F6 class would be 10 W class A ( but this needs a higher anode voltage than is available )
The nearest 6L6 class would be 18 W class A (but this requires around 360 volts for anodes, and 270 for g2 )
I have yet to determine if any frequency shaping has been carried out for each amp, as this will be on the radio chassis, which contains the preamp stages for each amplifier.
One of these days you'll be able to put all this information together in a book for a future generation.
Maybe a small manual, but if we can keep and restore some of this vintage knowledge, especially individual or bespoke designs then all the better. I have one rule in my house - no IKEA" We have to remember the golden age of hi fi was from 1955 to 1965, the quality of items built and the music we will never see again.
I met a guy who was throwing away his amplifier he built when he was 10years old - it had twin choke and px25s an amazing setup, he was ready to throw it in the skip....he worked for the bbc. It is a shame to lose this knowledge. We have to save some of this as this is expression and freedom and history.
Tech had been figuring out some of the valves in wrong place. Yet to confirm complete valve set.
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