Biasing Twin Reverb help.

[Sea Chief]

I think the avatar is pretty funny. But I am disturbed naturally.

You get our humour then. Chuck doesn't. It's the best avatar I've ever seen. Oh & it's a famous guitarist too, not me at all (of course it isn't.. who'd put a naked pic of themselves, up on any forum?! it only purports to be me -yes, the humourous idea).

20 full UK points, to anyone who knows who it is. Or the band. And played perhaps the best show I've ever seen too, mid 90's.

SC

[Sea Chief]

Anti-UK? I don't see that at all. Even if I squint and press my eye to one side until I see double, I still don't see it.

Just a feeling Mick.

Anyway, my lovely pro chap (who btw was -impressed- with my building, despite this unfathomable issue, which by now my work has pretty much been deemed not be the cause of) TOLD ME some mild noise does seem to still remain: this he told me on his final service checks after his work. I said that's ok, my fingers firmly crossed it might be a tiny irritating thing remaining.

The main thing is the HUMM has gone. So job done. What I didn't account for though, was the noises that linger, aren't tiny but prominent enough to give me jitters/ annoying enough to make me reticent about turning it on, especially sitting in front playing it.

SO. In conclusion then, to your collective credit, the conductive board theory -seems- to still hold water then, unfortunately for me (that is not by being proven, but by logic saying what else could this noise be due to, after such a thorough service).

So I have no option but to consider redoing the whole board. A winter project, if I can stomach it.

Thanks. SC

[g1]

Random pops or crackles could be preamp tubes, their socket connections, or their associated resistors (among other things). All my tube amps have the occasional pop or crackle, it's part of life with tubes. I don't worry about it unless it's more than occasional, and almost think of tubes as organic.

it's a famous guitarist too, not me at all (of course it isn't.. who'd put a naked pic of themselves, up on any forum?! it only purports to be me -yes, the humourous idea).

20 full UK points, to anyone who knows who it is. Or the band. And played perhaps the best show I've ever seen too, mid 90's.

You've spoke of them and this show before, so I'll wager it's Dean Ween of Ween.

[Sea Chief]

Random pops or crackles could be preamp tubes, their socket connections, or their associated resistors (among other things). All my tube amps have the occasional pop or crackle, it's part of life with tubes. I don't worry about it unless it's more than occasional, and almost think of tubes as organic.


You've spoke of them and this show before, so I'll wager it's Dean Ween of Ween.

20 UK points to g1! Impressed- it was just a pic up on their old website, which I found amusing & nicked it/ use it in all my 3 or so forums. Not Gardeners World mind.

51% dissipation.. & 61.2watts I've worked it out as, that's an average of 34mA. That's a result, no 135w horroshow, & under my 2x 40w Jensen speakers.

I'm sure though my chap went through all preamp tubes, as he replaced 2, plus the 2x 6L6 tubes. It's different to minor blips, it's an entrenched interference-snaffling scratchy sound, which comes & goes in sort of 'waves' ie 3 mins it starts up.. then peters out.. then suddenly back again louder, few glitchy pops in with.

I've recorded it so maybe I can get it up on YouTube & add a link to it. Such a shame after so much exhaustive efforts by my lovely chap. I'm 100% sure he couldn't do any more than he did. He did a huge,y comprehensive service, with him for months.

Thsnks, SC

[Helmholtz]

51% dissipation.. & 61.2watts I've worked it out as, that's an average of 34mA. That's a result, no 135w horroshow, & under my 2x 40w Jensen speakers.

Seems you calculated the 61.2W from tube dissipation.
That's not output power, but rather the power the tubes keep to themselves.
Output power is determined by B+ (or HT) and OT primary impedance. It does not depend on tube bias.
Output power needs to be measured using sig gen, scope and dummy load resistor.

[Sea Chief]

Seems you calculated the 61.2W from tube dissipation.
That's not output power, but rather the power the tubes keep to themselves.
Output power is determined by B+ (or HT) and OT primary impedance. It does not depend on tube bias.
Output power needs to be measured using sig gen, scope and dummy load resistor.

Aha, that was me wishful thinking then! I can ask my chap if he calculated the wattage then. Rift did so upon my renuild, at around 85w.

Actually I proposed an idea to my chap, of revamping the amp again: now, one 'issue' I have with it, is the sheer weight of it. The PT & OT are so huge the chassis actually struggles: both times it's come back via courier the xfmrs have pushed down warping the chassis. I have to bend it back & this hurts, so deformed it became. I'm sick of these 2 xfmrs: so, I proposed whether, in theory, I could change it into a Vibroverb.

If it's 2x 6L6 that's two less to worry about this annoying 'matching all 4' situation, if only a pair. And I've now got 2x pairs, so I'm set for good on that front.
Also the power lessens to 40w, the PT & OT might be sellable/ worth something (so cost might not be too bad), plus if a rectifier needed (can't recall) I have 2 spare sockets ready, with a nice gap to the power tubes.

The main reason I thought it possible, was noticing such a similarity in a DR I built, & this TR, on the main board.

Anyway, future idea. In meantime, what makes a pair, or a quad of power tubes "matched" anyway? Is it a case of expecting my 35mA figures to -all four- read exactly 35mA? If two read 35, but one reads 33, & another 34.. are these still "matched"? Or are there other factors at play?

Thanks, SC

[g1]

You wouldn't need 2 speakers for a 40W either. More weight reduction.

[pdf64]

The matching criteria for output valves is set by the vendor, there's no industry standard.
Pretty much universally with vendors marketing to the valve guitar amp market, it indicates that for a given combination set of electrode voltages, anode current will be closely similar, eg within +/-2% of each other. Preferably very close at other points too.
Hopefully the voltages used are typical of a valve guitar amp.

Matched that closely is unnecessary in my view, at least in fixed bias. Provided they're not idling too far apart, it shouldn't cause a problem. eg higher anode current not more than 1.5x the lowest current.

[Chuck H]

For the last twenty years I've been buying el84's in "batches". All same brand and typically same batch. Then I just match them for idle current. Usually withing 1mA. Never had a problem. There are much more extensive matching criteria measuring current and gain when conducting signal, etc. I've honestly never audibly recognized a discordant imbalance or had any tube life or failure problems doing it my way for perhaps two dozen sets of tubes. Both pairs and quads.

[Sea Chief]

You wouldn't need 2 speakers for a 40W either. More weight reduction.

I was thinking maybe make a 2-10 baffle, pair of alnico jensens: alnico's seeming lighter to me. Sell these two 12" jensens. Bin the chipboard heavy baffle, make a 9mm birch ply one, prolly slightly 'springy' too being a bit wider than a Vibroverb.. 2" wider iirc.

Anyway I was really asking for opinions on whether my Vibroverb idea has merit.

SC

[Sea Chief]

The matching criteria for output valves is set by the vendor, there's no industry standard.
Pretty much universally with vendors marketing to the valve guitar amp market, it indicates that for a given combination set of electrode voltages, anode current will be closely similar, eg within +/-2% of each other. Preferably very close at other points too.
Hopefully the voltages used are typical of a valve guitar amp.

Matched that closely is unnecessary in my view, at least in fixed bias. Provided they're not idling too far apart, it shouldn't cause a problem. eg higher anode current not more than 1.5x the lowest current.

Hi Pete,

So with my 2 pairs of 6L6's, the outer two of V7 & V10 being JJ's at 36mA, 35mA.. & my inner two of V8 & V9 (tung sol) at 32mA, 30mA: are these very badly matched? Not matched too well? Not bad? Or even -just about- kinda acceptably matched ?

Having 4 is nothing but a royal pain in the butt afaict. And I've got two amps with them in!? Thankfully the ac30's well behaved, so far, fingers firmly crossed.

SC

[Justin Thomas]

Chief,

I'd call thet pretty good. Just put the higher 2 in the outer positions & the lower in the inner positions (or vice versa). That way each side of your OT sees roughly the same current.

If you put one matched pair on one side & the other on the pther, I bet you'll get some hum. Try it & see - won't hurt anything to try for a few minutes to check the effects.

Jusrin

[Chuck H]

Lightening the Twin with a thinner baffle and smaller/lighter speakers and then reducing power to two tubes is a decent start, but... It's really that heavy cabinet and that heavy iron that are still the lynch pin in this plan. By the time you replace the baffle, speakers and transformers (the rest of the huge cabinet not withstanding) you will be too far into the monetary aspect to make good and you'll have decimated resale value on the amp as original. Since the amp has already been a pain in your @$$ I might suggest selling it and buying a more suitable mod platform for your goal. Reverse engineering the Twin at this point seems to have too many negatives. JM2C. Uncle Chuck.

[pdf64]

So with my 2 pairs of 6L6's, the outer two of V7 & V10 being JJ's at 36mA, 35mA.. & my inner two of V8 & V9 (tung sol) at 32mA, 30mA: are these very badly matched? Not matched too well? Not bad? Or even -just about- kinda acceptably matched ?

They look to be fine.
I agree with Chuck's analysis in post 58; it seems wrongheaded to mod the amp to be a Vibroverb, better to sell it on now it's working ok and get something better suited to your needs.