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.
Hello All, sorry about the delay in updating you of the current repair of the amplifier! Here is what the Tech has stated...
"An update and a few pics for you. I finished changing all the capacitors. Power was then applied in stages. The stabilisers made some strange noises and one part lit, but the other one was arcing due to the loose base having torn out the lead out wires. I installed a couple of NOS S130s and powered up again. The slow start relay was giving a few problems, so this has now been wired out. Although a good idea in theory, with valve rectifiers giving slow turn-on, it’s not really necessary.
The voltages were all checked but the valves seemed out of balance, and they are a mish-mash of types,/sizes so before going any further I treated it to NOS pairs of 6L6 and 6F6 for the different amplifier sections, as this is what I believe was originally intended
During testing, ocassional ‘arcing’ could be heard. I could not see an obvious reason for this but to me it sounded like AC problems. so it was powered down and all the voltage selectors and fuse holders were double-checked. It was while doing this I noticed the state of the wiring to the primary of the large mains transformer. This had been wired in rubber cabling for the voltage taps and all three were crumbling away ( see photos ) As I followed the leads back above chassis they literally fell apart in my hand.
As only the 240v wire is used, this was rewired with new cable, and the rest double heat-shrinked to keep then out of harms way. I was concerned that they may break off at the transformer whilst carrying this out, but the wire itself was good and strong, which is more than can be said for the insulation !!
Tthis seems to have fixed the problem. The whole chassis now powers up without fault and gets remarkably warm quite quickly. There is a hum response when tapping the input connections, and all the voltages look reasonable. The output stage balance was re-adjusted for the new valves and they are drawing a fair bit more current, which is to be expected as their emission is ‘as new ‘ To my surprise , the original paper in oil coupling caps seem fine with no leakage
The next stage will be the radio chassis. There is so little ‘gain’ on the main chassis ( inputs direct into phase splitter ) that it will need the preamp sections of the radio chassis to get any drive and see how many watts each amplifier actually outputs. There is still the question of the two missing valves. There already seem to be more than enough valves/stages on the radio chassis for this set up so it’s hard to fathom what two more valves are likely to be adding. All will be revealed soon, hopefully. "
So amplifier completed! So we need to find what the two missing valves or maybe they are not needed...
Will let you all know what happens next!
I have also been sorting out the turntable so will post a video for everyone once we have totally finished. I am so looking forward to the complete restoration!... keep you posted!!!!
Ah-ha, crumbly natural rubber insulation. Anyone who has dealt with vintage electronix of a certain era are familiar with this nuisance.
I remember well a certain Swiss made Mediator table console radio with the sidewiper Philips tubeset. Replaced every wire in that thing.
I think people fixing these amplifiers etc are pretty amazing. I really wouldn't know where to start so i am very happy Andy the Tech looks to have it under control. He told me the missing valves are puzzling him still so will look forward to his next update regarding connecting the two to get some signal. Fingers crossed!
Figuring out location function and socket connections should give hints of tube type vs location. Certain types are typically used for RF and IF amplifier stages for instance. Others mainly used as audio gain and audio output power tubes. Rectifiers tend to be self evident by connection and location.
Yes we had a big discussion but tech says at the moment there seem to be enough valves already but I am sure he will figure it out. In the meantime the turntable is being setup for the player so will post more pics as soon as I can...
It has been over a month now since the work has started on the amplifier. Current update is the AM Amplifier is being worked on and I hope to get a response from tech next week. Lets hope he can get it working as am i close to setting up the turntable (just a few choices regarding tonearm, board material and cartridge). Mono or stereo etc. Lets see what next week brings as i am really missing playing some vinyl esp with the little kids! lol
Ah, rubber wire, common stuff in the late 30s here but some probably was around shortly post-war. I have something from the late 20s that is full of it, probably a very cutting edge radio in it's day, now its just something I need to rebuild eventually.
We have some audio ! I have powered up the radio chassis and there is at least audio output appearing on both amplifiers. I need to change a few capacitors which are measuring ‘leaky’ but nothing has smoked or gone pop at least! All the dial lamps are blown, so they will need replacing.
Someone has 'wired out' the left and right lower panel controls which allow volume to be adjusted to each amplifier, in favour of the control on the side which adjusts both at once. I will probably reinstate the wiring to these, so the levels from each speaker can be set separately. The one on the side will then be the master volume.
There’s no output from the radio tuner section, so this still needs to be investigated. Probably down to the caps again.
More soon !
" I Can see the light "
Yes, we have so much to do with so little time. Luckily I found i tech that actually knows what he's doing so i am happy i am not breaking things before trying to fix them again lol. Personally i am trying to declutter and just keep the nice things that work! well thats my new motto anyhow.
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