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Excelsior modification

Discussion in 'classic' started by djp150, Dec 24, 2021.

  1. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    No I'm not an owner. I'm a designer and repairer of amps etc and the "Jez" mentioned above.

    I can only really comment on what is in the diagram.... my crystal ball for checking out yours from here and seeing whats different is a bit dodgy these days:rolleyes:

    According to the diagram; V1 has it's own heater supply of 6.3V AC which is floated at HT voltage. The cathodes are at HT level and this would put strain on heater/cathode insulation otherwise. ie the heaters here run from 6.3V AC BUT the whole shebang is floated at say 250V DC above ground. V1 is not a regulator or anything like that but is wired simply as a pair of diodes in series... some voltage will be dropped across these but otherwise I can see no purpose to V1 other than to prevent HT being applied to the rest of the amp until V1 warms up.

    All other valves have heaters supplied from 12V DC from the regulated supply. A half wave voltage doubler takes 6.3V from the transformer and doubles it... in fact there should be about 16V ish on the smoothing caps. It then goes through a 3 pin voltage regulator to give 12V. Valves such as ECC81/82/83 have centre tapped heaters and can be run from 12.6V applied to pins 4 and 5 or 4 and 5 can be connected together and 6.3V applied between there and pin 9. If you have the rest of the schematics for the whole unit it would be useful!

    The regulator circuitry for the heaters will be bloody obvious if present and correct as apart from the 12V 2A regulator on a heatsink there will be 4 large 10,000uF electrolytics!

    I'm rather busy and generally turning down work ATM but this is interesting enough and not to long a job so I could fit it in as a quicky if need be.
     
    eastone likes this.
  2. djp150

    djp150 Member

    The 6.3v output from the transformer that supplies the separately boxed preamp is working fine . As you say I have around 17v prior to the regulator and 12.8 coming out of it and around 6.1v at the heaters on each valve in the pre-amp .
    I will try to load the other circuit later after the relations have disappeared. I will also send a photo of the power supply showing what’s connected and what isn’t . And thanks for the offer -it’s good to know that if I cannot do it there is someone around who knows valve circuitry. I’m sure that it’s virtually a matter of a couple of wires to get it as the manufacturer designed. At one time I had 2 of these and how I wish I still had the other to guide me .
     
  3. djp150

    djp150 Member

     
  4. djp150

    djp150 Member

    Ok , I get it now. Takes time when you are in your 70s ! I run the 6.3 output from the transformer directly to the heater of the valve and take a 100ohm resistor from pin4 and pin 5 to pin8 . So my only question is what wattage should the 100 ohm resistors be . Everything else on the board ties in with the circuit diagram .
     
  5. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    1W will be more than good enough. Be certain that it is as the diagram in that there is definitely a completely separate 6.3V winding on the transformer for JUST V1
     
  6. djp150

    djp150 Member

    Yes the 2nd set of 6.3 windings are unused which is strange but for some reason on my box it was decided to control the HT by solid state even though the board is the one that conforms to the circuit I posted -strange -I thought at 1st that the 6.3 output was probably not working but it is . Anyway thanks again . I will get some resistors -wire up and hopefully all will be fine ‍
     
  7. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    EH??? Both windings should be used! You know where I am if it blows up....
     
    Last edited: Jan 10, 2022
  8. djp150

    djp150 Member

    Yes I do realise that. I think I understand it now . I must admit I couldn’t understand why the circuit was suggesting those 2 resistors should go to pin 8 HT but you’ve explained it to me and it’s also mentioned on the TNT website on the excelsior page . Anyway nothing to lose as I nearly ditched it last week . I was going to use my transistor pre to drive my Williams Hart monblocs. I’m in no doubt there are a few on PFM who will have a good laugh if I have to come back to you with a burnt out pre. I did look to see if you are local to me and if you had been within 20 miles or so I would have brought it over -but I’m in Wolves and I think you are up Sunderland way . Anyway thanks I really do appreciate your help.
     

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