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Old 3rd April 2020, 04:01 AM   #1
vernonb is offline vernonb  New Zealand
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Default TSEII with a Hybrid brigde?

Hello
Iím fairly new with tube amplifiers (built a few simple ones before)


Iíve just started assembling a TSEII and stupidly made some assumptions about the transformers I have available to me, I did have a search around the forums before I ordered everything(I already had the transformers) but you know how it is, you find the tricky posts beyond your ken after you have started to build it (I guess better than when you turn it on for the first timeÖ..)


My power transformer has no CT and I was planning on doing a hybrid graetz bridge to get a ground to the star point on the board and the voltage required, it seems to sim fine in PSUD2


However, Iíve come across some posts from Tubelab warning against using protection diodes on the rectifier legs because it blocks the neg voltage supply.
The Diodes in a hybrid bridge are not configured the same way but I thought Iíd better ask in case it still blocks the neg voltage supply, and if it is a problem? has anyone got a solution that preferably doesnít involve a new power transformer? (Pretty hard to access down here at the best of timesÖ.)


Thanks (Iím sure I will have more stupid questions in the near future)
Vernon
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Old 3rd April 2020, 12:34 PM   #2
Tubelab_com is offline Tubelab_com  United States
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This solution does not have a path for current to flow in the negative voltage supply since neither end of the power transformer ever swings negative.

Do you have a second transformer available with a secondary of at least 115 volts at 50 mA or so? If so you can build a small separate supply for the negative voltage.
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Old 4th April 2020, 04:05 AM   #3
vernonb is offline vernonb  New Zealand
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Thanks George
I probably should be a bit more specific about the transformer now I know itís the wrong oneÖ Itís a Lundahl LL1648 and I think I can tap off between the two windings to create a CT but that only gives me 175VAC using the tube rectifier and no bridgeÖ.(specs below)


Transformers are pretty hard to access/source here, we are in full lock down (most mail/couriers as well) and my main spares box is inaccessible (not sure thereís much useful in it anyway)
Iíd rather not put more iron on the chassis but if itís the only way (Of course I could just waitÖ.) ;-)


Available here and now is a 250-0-250VAC from an old sony RTR, looks about 100-150VAÖprobably overkill, but I may be able to at least do some testing with it (or is the voltage way to high?)


A 6+6v 24VA (2x 115 primaryís) encapsulated transformer, I wondered about putting it backwards on one of the Lundahl filament taps, But Iím thinking its maybe too small? And the so are the filament taps (small)?


Other potentially stupid ideas that probably still wonít work for the same above reasons (current path) but Iím a bit dense when it comes to transformers


Use the LL1648 Centre taped with a sand voltage doubler first to get ~350VAC and then into the 5ar4 for a slow start, there must be an obvious reason not to this because I canít find any examples, maybe itís too ugly


Do a full bridge off the Lundahl's serised secondaries and do the neg voltage between the CT and one endÖ. I donít think this can work though as the ground path gives me a headache, see the drawing below from the internetÖbut it is two positive suppliesÖ..


Thanks so much for your help and time
Vernon
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Old 7th April 2020, 06:09 AM   #4
OldHector is offline OldHector  Sweden
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Isn’t the link on the secondary 6 -> 8 a center tap?
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Old 7th April 2020, 01:31 PM   #5
Tubelab_com is offline Tubelab_com  United States
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Quote:
Isnít the link on the secondary 6 -> 8 a center tap?
It is, but grounding it will give a 175 - 0 - 175 Volt secondary. This could be used to generate +/- 225 volts DC or so, and could probably be used with a full wave voltage doubler on the positive side for + 380 to 400 volts and - 225 volts.

I'm sure that it's doable, but I can't quite wrap my head around the idea right now either. Look at the circuit that I used to generate +160 volts and +320 volts from a single 120 VAC transformer in this guitar amp. I'm thinking that the CT should be grounded and the HV secondaries connected directly to the negative voltage diodes. D1, D2, D3, D$,C24, R61, and C23 would be eliminated as that was for the +160 volt circuit.

The circuit comprised of D5, D6, D7, D8 C21, C22 and C20 make up the voltage doubler. D5 and D7 would be replaced with the on board 5AR4 and C20, would be the on board filter components.

Note the guitar amp shown in this schematic has been working for several years. I have NOT actually tried what I proposed, or even tried to simulate it. Most of my ideas do work, but often require some tweaking, but a few just blow up.

Quote:
A 6+6v 24VA (2x 115 primaryís) encapsulated transformer, I wondered about putting it backwards on one of the Lundahl filament taps, But Iím thinking its maybe too small? And the so are the filament taps (small)?
That's probably the safest bet. The current required on the negative supply is about 25 mA and the 150 to 175 volts of negative DC voltage is plenty if you replace R6 in the TSE-II with a jumper wire. 25 mA at 150 volts is less than 5 VA so both your transformer windings are plenty big enough. It's also the easiest path since it can be dome without hacking up the board. Just lift the ends of the on board diodes and solder wires from those ends to the small transformer. Tack the pair of diodes for your HV bridge on the bottom of the board somewhere.
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Old 7th April 2020, 11:38 PM   #6
vernonb is offline vernonb  New Zealand
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Thanks for responding guys
After a couple of days of brain hurty research and reading Iíd mostly come to the same conclusion, 6v transformer was the most straight forward, at least until I can get a standalone mains supply aux transformer.
Iíve been fiddling around with the chassis layout to make it all fitÖ.

Iíd come across your Guitar amp schematic in my fevered researchÖ but grounding the CT was just making me nervousÖ.
I will try it I think when I have some more disposable parts to kill, rather than the LundahlÖin fact letting the smoke out of anything just now would be a downer

In terms of the 6v transformer, I guess Iím running it at 115-CT-115ish and grounding the CT at the board star ground?

Thanks for your time and help
Vernon
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Old 8th April 2020, 02:15 AM   #7
Tubelab_com is offline Tubelab_com  United States
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Quote:
I guess Iím running it at 115-CT-115ish and grounding the CT at the board star ground?
Yes, that should give you about -150 volts, so you would need a jumper in place of R6
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