trinityamps.com

I have about 370V on my EL84 pins 7, as I've noticed is also the case with a few others commenting on the forum. The TC-15 literature gives a significantly lower value, 295 I think. I've been running JJs with no issues at my 370V.

But I'm wanting to replace my JJ EL84s with some NOS Amperex, which are recommended to not be used at more than about 340V. Anyone tried something similar to this with problems resulting?

Am I destined to burn out my nice Amperex tubes? Could I decrease that voltage to say 340 without losing the great sound of the TC-15?

Thanks!
Jim

Jim,

If you want to be really safe, you can insert a 5W diode in series with your B+ . For this amp, up to 30V Zener could be used safely with a 5W rating.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I am using a JJ rectifier, so another option might be to replace that with a NOS 5AR4. Wouldn't that reduce the B+ voltage a bit? Another issue with my setup is my house voltage, which runs high, about 125VAC. I had forgotten, but checked and my PT voltages are 310 on each side, instead of the 290V the TC-15 literature states is more normal with this transformer. Since I can't easily reduce my house voltage, would it be worth trying a NOS rectifier before adding a Zener to the circuit?

Not sure how much voltage the NOS would drop. Check first. A 5W zener is quite cheap in comparison.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Just FYI...my line voltage is around 123v. I built a TC-15 for Fortis a while ago & the voltages were very close to spec. Don't recall what rectifier was in there though.

IIRC that's only slightly higher than the Vox and Matchless amps that the TC-15 is closely related to. Somewhere around 330V to 350V is probably a good voltage range to aim for. 370V is a little high in any case.

Don't forget that all the voltages given in tube specs are relative to the cathode. So let's say you bias your amp with the cathode at a fairly typical 12V. That means you can run those tubes safely up to 352V on the anodes.

In my experience most EL84s will sound richer at the slightly lower voltage. You don't want to go too far lower than that, or the amp will start to lose crispness and headroom.

I thought that trick only applied to NOS 5Y3s vs Sovtek 5Y3s. I might be wrong about that though...

What's really important is that your heater voltage shouldn't be more than + or -10% out from 6.3V AC, or your tube lifetime will be reduced. In other words it should be below 7V AC. FYI you can in effect reduce your house voltage, by putting in a Vintage Voltage Adapter - www.geofex.com/Article_Folders/vintvolt/vintvolt.htm Just bear in mind if you use one of these, your heater voltage still mustn't be allowed to go more than 10% below the 6.3V AC norm.

+1 However, I would feel safer with a 10W or 12W zener (placed in line with the B+ rail after the first filter cap). Either that or divide the voltage drop between a couple of 5W zeners. So for example you could have two 9V zeners in series for an 18V drop.

HTH

_________________
Great tones come in small glass jars!

Thanks, Coco, Emohawk, and Zaphod. Very helpful info.

I did check my heater voltage and it is high, 7.94V! Ouch. Obviously, time to get that Zener in place. But one more question about where exactly to place it.

Zaphod, you suggest "in line with the B+ rail after the first filter cap." Do you mean by "first filter cap" the 32/32 can cap? i.e., place the Zener in line from there (the standby switch side of the can) to the OT? I'm thinking the resulting lower voltage into the OT would then lower the OT output voltage to pins 7. But I always forget how the electronics work unless I've recently read up to refresh my aging brain.

If I decide to try a NOS rectifier sub, I'll measure the B+ and post what I find. That might be helpful for folks to know.

Sorry to bombard you guys with more questions, but here's one more related to this topic:

One site states that the 5U4 rectifier tube would drop the 5AR4 voltage by about 20V. That would seem ideal for my situation - plug and play. But I believe the 5U4 also draws more filament current, which could be a problem for the PT - unless that PT can take it. There may be other issues also - I am a bit reluctant to quickly swap out the recommended 5AR4 without some good advice first.

Coco, Zaphod, others? Any recommendation about that match up? Anyone using a 5U4/5931 in their TC-15?

Thanks,
Jim

No, that will only reduce the B+ voltage without affecting your heater voltage in any way at all. You need to put in the Vintage Voltage Adapter.

You can pretty much put the zener(s) anywhere you want between the first filter cap (the side of the 32+32uF that goes to the rectifier) and where on the OT centre tap wire gets connected. However, it looks like it's not a zener you need.

And that won't help the heater voltage either. So like I said, you really need the Vintage Voltage Adapter to reduce the whole incoming AC supply voltage.

_________________
Great tones come in small glass jars!

How embarassing - I read the wrong numbers from my TC-15 schematic. Actually my heater voltage is good, within range - 6.6V. Thus, I'm back to thinking a different rectifier is worth a try.

The 5U4 should put out about 20V less than the 5AR4. But I'm wondering if there might be other issues to consider by switching to that tube, which is not the one specified for this amp. Any thoughts or experiences with this kind of substitution?

Thanks for all the good help!
Jim

A 5U4G has an internal impedance of 195 ohms vs the 175 ohms of a 5AR4. So it will drop a little more voltage, depending on current draw, but not a whole deal more. It's a high current tube so no problems there, although I suspect it may not give you as much of a drop as you want. But why not try a 5Y3 (non Sovtek). It has an internal resistoance of 480 ohms and should therefore drop a more useful amount of volts, and has the correct ratings for a 15W amp such as the TC15. You will also get more sag with a 5Y3 than with a 5U4G or 5AR4, which will give the amp more of a vintage feel. OTOH if you use a zener (or two), you will have a fixed voltage drop regardless of how hard the amp is working.

_________________
Great tones come in small glass jars!

A 5U4G draws 5V @ 3A heater which is 1A over the spec on the PT 5V tap. So you can try it, but it may overload the PT. I would go with the Zener. I think the 5Y3 may be to much sag for a TC15 unless you retro fit a choke.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks, Zaphod and Coco.

I tried an RCA 5Y3 last night, and it was not bad - moved the amp over in the direction of a Deluxe - but it lost some of the sparkle and clarity. I think I will go ahead and install the Zener. I'll report back after that is done, in case anyone else has too high voltage on their EL84 plates and wants to go with something other than the JJs.

Regards,
Jim

Or how about just fitting a VVR?


The resistance in the choke would also drop the B+ voltage even further.

_________________
Great tones come in small glass jars!

A VVR reduces B+ voltage, right? So it would be in effect like adding a variable Zener to reduce the plate voltage to the power tubes. Does it affect the circuit in other ways? I installed one on my Trinity Deluxe (and like it very much), but can't remember the details of how it works.

Coco, re the PT you supply in the TC-15 kits: The schematic actually shows 5V and 3A to the rectifier heater. Are the specs only 2A, though, as you say? You're certainly the one in the know about that!

Oh, another question that occurs to me: If I am reading the circuit correctly (and I might not be!), it would seem that placing a Zener near the standby switch would reduce voltage to ALL the tubes - power tubes AND preamp tubes (since the preamp tube plates take their voltage from the various large filter caps). Wouldn't placing the Zener AFTER the cap can (and the 1K5, 5W resistor) keep the higher voltage going to the preamp tube plates, but reduce the voltage only to the EL84s? That might be what I want, since I don't think there's a problem in running the preamp tubes with my current voltages.

But what do I know? Not as much as I'd like...


Thanks again.

Exactly!

Not that I can think of, apart from the usual benefit of cranked tones at low volume levels.

It's actually a good idea to reduce the voltage on all the tubes by a proportionate amount, so that they all come into line with what they should be.

_________________
Great tones come in small glass jars!

I did ask Heyboer when we started down this path 3 years ago and they said not 3A on the 5V, maybe intermittent use but not full duty.

So you're back to a Zener or VRM which you can install to just feed the power tubes or the entire board. If you just want to feed the power tubes, you will need to modify the board so the preamp circuit run independently.

I would feed the whole amp [same rec as Phil] but then you might need to modify the input to block DC. You dont need to do that mod if you just feed the power tubes but you will need to use the master more to get the same tone at lower volumes.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Just to clarify, my recommendation was in the context of using a zener (or two). For a VVR, where you can go down to very low voltages, you may indeed decide you only want to regulate the power amp, or the power amp and PI. Then you would need to do some stuff with diodes to isolate the power of the sections you didn't want to regulate.

_________________
Great tones come in small glass jars!

Thanks again, guys.

Unlike some others, I don't find that using the master volume takes away much tone from this amp (may be my old ears), and in fact I usually just adjust the channel volumes and leave the master out of circuit. I tend to like the TC-15 running clean, and add dirt when I want it with a Blues Driver or Twin Tube.

So rather than installing the VRM, I'll just try the zener(s) to reduce B+ for the entire amp circuitry. In this case, I shouldn't have to worry about separating out the preamp from power amp sections, correct? Since my B+ is really high, I want to reduce power by 5% or so for everything fed from B+, if I'm following your explanations right.

I'll report back once the zener(s) are installed. Then we can close this topic, which must be getting tedious to everyone by now!

Jim

Not at all. Good discussion on the options.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps