Implimenting a tubeamp design board startly

[pjd3]

Hello all,

Moving into my 3rd tube amp I will be adding a bit more experimentation and light design. My first 2 builds were largely done by the schematic with some mods (ppimv, raw mid, variable NFB) but now is time to become more intimate with whats happening inside these circuits - could sure use some guidance as I go.
There is a board set up with PT, breadboards, tube socketed pcb's and am looking to start with getting the HV section going first.

Being an experimental design station of sorts, I think it would be smart to fuse any path that could be a source of damage to any costly component. I did the math for the PT primary and know a 2 amp fuse would be a good choice for the particular PT being used for this project but, How about a fuse for each of the HV lines coming off the secondary? Seems as though if the tube rectifier got into trouble it could pull alot of current from either end of the High voltage coil. Ya think? With this HV secondary rated at 170mA do you think a 1/4 amp fuse in line with each end of the HV coil would be smart for protecting the HV coil? Or perhaps, would say, a IN4007 diode be enough? Or both? If you were trying to be cautious and hyper-vigilant how would you address that? I'm a careful and ruminating guy but, I don't mind bone-head proofing myself in case of a senior moment.

Thanks!
Best, Phil Donovan

[pjd3]

Just to mention, I was just over at the Hoffman forum and came up with a thread talking of the same topic I brought up about protecting the PT HV from over current. A few of the posters were people found at this forum. I come away with the thought that using both diodes and fuses sure wouldn't hurt on the way to the rectifier on both legs of the HV coil.

I guess now its just a matter of choosing the best value/type fuse for that application. For primary fusing, I was directed how to find the volt/amps of each secondary coil, add them up and divide that by the primary voltage. That would give the max current ever expected in the primary, then just double that for the fuse value. I suspect but don't know for certain if that tactic would somehow apply to any system in a tube amp (double the max expected current to find a reasonable fuse value).

Thanks!

Phil Donovan

[R.G.]

I did some thinking about protecting expensive tube amp parts back in the early 2000s, and came up with some ideas on that. I wound up doing an article on that for Premier Guitar. i think it's still on their web site.

Transformers are tough, and they die primarily (heh, heh...) by overheating decomposing the insulation between turns. Stop the overheating, you protect the transformer. Internal heating in AC mains transformers is nearly all from current heating the wire resistance, so limiting the current does it in nearly every case. Multi-tapped windings complicate this a bit, so you need one fuse external to the transformer for each isolated section of copper wire. In a center tapped winding like the HV, you need one fuse on each of the outer sides. A fuse on the heater winding is also a good idea, since a shorted heater won't necessarily open the mains fuse in all cases.

Some other ideas i this vein were to back up each plate of a tube rectifier with a solid state rectifier capable of doing the rectification all on its own. The SS diode would prevent a shorted rectifier tube from killing the filter caps or the transformer (in case the trannie didn't have fuses). Less applicable to your setup is the idea to change the filter caps to motor run film types that will never self-rot like electros do. Then there is the idea to limit peak currents out of the rectifiers to prevent current surges in case of a filter cap or power tube failure. There's a current-limit clamp circuit at geofex.com showing how to do this. It stops current surges from blasting the rectifiers at power on and going off standby.

[pjd3]

thank you R.G.,

some of your suggestions are new to my ears and certainly sound like they deserve a good look at.

Apart from the practical stand of protecting the amp from aging and compromised parts I'm also going to be looking to protect the design board from "ME" ! I bet fuses on every coil leg would come in handy here.
Should some leads be inadvertently touching on the bread boards because I didn't notice is something that could happen if I turn bonehead and eager to try something later in the night when tired and half blind (of course, we know what the true solution is there).

I will be looking into your recommendations and article. They sound like just good ideas from the non corner cutting aspect of building your own amps for performance and reliability.

thank you RG,
Best,
Phil Donovan

[pdf64]

An excellent method to protect your amp from bad / badly implemented, ideas is to be strict about using your light bulb limiter every time it’s initially powered up after you’ve been a-tweaking.
As transformers have an annoying habit of nobly sacrificing themselves to save the life of their close comrades, the mains and secondary fuses.

[pdf64]

I did some thinking about protecting expensive tube amp parts back in the early 2000s, and came up with some ideas on that. I wound up doing an article on that for Premier Guitar. i think it's still on their web site.
...
Some other ideas i this vein were to back up each plate of a tube rectifier with a solid state rectifier capable of doing the rectification all on its own. The SS diode would prevent a shorted rectifier tube from killing the filter caps or the transformer (in case the trannie didn't have fuses)...

See https://www.premierguitar.com/the-immor ... lifier-mod
Firstly, thank you very much for these articles and the geofex site, I've found them extremely useful and thought provoking over the years.
However I'm a little squeamish about recommending the use of 1kV diodes to back up a valve rectifier for HT winding voltages of much over 550V, eg a Vox AC4. Certainly even with a Champ etc, I'm reaching for 1k2V or higher rated diodes.
I suppose that back when you first wrote this stuff up, maybe 30 years ago, the availability diodes with reverse voltage ratings >1kV was pretty sketchy, I guess we've got SMPS to thank for more / better higher voltage components. And hindsight is 20/20

[pjd3]

What I got so far (but not finished1)

I did the math for line voltage divided by the sum of secondary V/A and came up with around .92 amps. I heard that doubling that amount can be a reasonable fuse rating for primary.

For the HV lines my https://www.edcorusa.com/xpwr096 power transformer states an HV current of 200mA center-tapped so I was going to bring a .25 amp fuse into each leg unless someone was to prudently advise differently.

I would be happy to set up a "current lamp". Would someone know of a "bulb rating chart" for choosing the best bulb for the voltage/current/power of the amp in question? I imagine not just any bulb would be suited for the large range of amp powers.

Thank you so much for your valuable information. I may live more than a day with this new amp "design" (which is really me starting with a hi-fi amp circuit and modifying (corrupting) it for tone and output variations/optimums for a particular instrument I built.

Best,
Phil

[martin manning]

For the HV lines my https://www.edcorusa.com/xpwr096 power transformer states an HV current of 200mA center-tapped so I was going to bring a .25 amp fuse into each leg unless someone was to prudently advise differently.

Unlike primary fuses, HT fuses are not time delayed, so they will need to be oversized to survive the initial surge. I would go with 500 mA. See: http://www.valvewizard.co.uk/fuses.html

Would someone know of a "bulb rating chart" for choosing the best bulb for the voltage/current/power of the amp in question? I imagine not just any bulb would be suited for the large range of amp powers.

A good starting point is to use an incandescent bulb with wattage approximately equal to the amp's output power. I recently rebuilt my bulb limiter: https://ampgarage.com/forum/viewtopic.p ... 20#p424920

[pdf64]

Unlike primary fuses, HT fuses are not time delayed...

Dunno that’s a given?
I can think of amp models that specify T type HT fuses, inc many Marshalls.