TSEII with a Hybrid brigde?
 

Hello
I’m fairly new with tube amplifiers (built a few simple ones before)


I’ve 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 time…..)


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, I’ve 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 I’d better ask in case it still blocks the neg voltage supply, and if it is a problem? has anyone got a solution that preferably doesn’t involve a new power transformer? (Pretty hard to access down here at the best of times….)


Thanks (I’m sure I will have more stupid questions in the near future)
Vernon

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.

Thanks George
I probably should be a bit more specific about the transformer now I know it’s the wrong one…:) It’s 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 bridge….(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 there’s much useful in it anyway)
I’d rather not put more iron on the chassis but if it’s the only way (Of course I could just wait….) ;-)


Available here and now is a 250-0-250VAC from an old sony RTR, looks about 100-150VA…probably 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 primary’s) encapsulated transformer, I wondered about putting it backwards on one of the Lundahl filament taps, But I’m thinking its maybe too small? And the so are the filament taps (small)?


Other potentially stupid ideas that probably still won’t work for the same above reasons (current path) but I’m a bit dense when it comes to transformers:confused:


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 can’t find any examples, maybe it’s too ugly


Do a full bridge off the Lundahl's serised secondaries and do the neg voltage between the CT and one end…. I don’t think this can work though as the ground path gives me a headache, see the drawing below from the internet…but it is two positive supplies…..


Thanks so much for your help and time
Vernon

Isn’t 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.

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.

Thanks for responding guys
After a couple of days of brain hurty research and reading I’d 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.
I’ve been fiddling around with the chassis layout to make it all fit….

I’d come across your Guitar amp schematic in my fevered research… but grounding the CT was just making me nervous…. :confused:
I will try it I think when I have some more disposable parts to kill, rather than the Lundahl…in fact letting the smoke out of anything just now would be a downer

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

Thanks for your time and help
Vernon

Yes, that should give you about -150 volts, so you would need a jumper in place of R6

Bias issues
 

I kept meaning to update this thread with some pics but still still never quite got the chassis finished.


I’ve been using my amp for about 5 months, most days, probably less than 500 hours.
A couple of days ago something went wrong with it and I have spent a day fiddling with it trying to get to the bottom of it


I noticed on powering off that the meters were reading wildly out of range, V3 had pegged and V4 was showing nothing at all practically, It had been idling for about an hour so I’m not sure if there was an “event”


The next day I went through a full checkout and everything seems fine except I can no longer get the bias stable at all, Whichever channel I’m adjusting the bias on, either carries on rising or falling depending on which way I was moving the pot (I think, could be fooling myself ) and of course the other channel reciprocates, I haven’t been methodical enough to isolate it to one channel or tube most because the interaction was making my head spin….so swapping the tubes over hasn’t seemed to prove much….


Is this just a form of tube death? I’m a bit sad if that’s the case after >500 hours….



Or may it be something else? Coupling cap, or a mosfet?



I haven’t got any spare 300Bs, and should have some anyway (at least for trouble shooting) but if it was a less spendy component that was broken, that would be nice, one of the reasons I don’t just rip into it and replace stuff in hope, is the silly chassis I made which makes access a wee bit painful, and if it is tube death I’d rather not spend a few days chasings shadows and waiting for parts to arrive


Are there some other tests I can try before I dismantle everything and/or spend money on tubes?



I have measured grid bias control on the outputs (without tubes) at ~ -18 to -79 both channels


I’m running a few non-standard bits, A separate 25VA 110v transformer and small bridge rectifier for the neg bias,
Russian teflon’s as coupling caps,
and the PSU is motor run caps only, all off board


Thanks
Vernon

its interesting that you are having issues with bias continuing to move in whatever direction it was last adjusted. Ive been experiencing something very similar on my tseii running gold lion 300b tubes. Mine appears to be only one tube and followed when i moved the tube between sockets. Seems to me that it is a tube issue. Ive been running my amp pretty regularly for about a year with the same tubes which shouldnt be enough to wear out a tube.

Thanks for the info brl0301

Coincidentally! my tubes are also Gold lions
Does your bias settle eventually?

Mine doesn’t seem to stop drifting till I crank the pot the other way
And with the other channel doing the same but in the other direction…its been confusing as to which tube is tube/channel is causing it,

What are your operating points for your 300b if you don’t mind me asking?
My operating point was ~405v at 68ma and a 3k opt….


Vernon