VBIGC troubles...

[Joe Rasmussen]

Originally posted by elizard
One question with the schematic. Can it be used w/o the volume pot?
So basically use it as a power amp (like i'm using my IGC right now)?

You sure can! Just use 47K to ground where the pot is. This will ground the input and also define its Z.

Joe R.

For anyone who want to see my photos from the European Triode Festival:

http://members.ozemail.com.au/~lisar...3/ETF_2003.htm

[elizard]

Originally posted by Joe Rasmussen


You sure can! Just use 47K to ground where the pot is. This will ground the input and also define it Z.

Joe R.

Thank you Mr. Joe.
I've a couple more questions for you if you don't mind.

First off, I think I know the answer to this, but I'll ask to be sure. It doesn't matter if the gainclone chips run off of a lower voltage supply than the tube itself does it? (i've an IGC going now, and i would re-use its power supply with a couple additions for the vbigc)

Secondly, i understand the answer to use a 47k pot to ground. Should I be using this in my IGC as well? In my igc i basically left out the pot completely, so i guess there's no connection between signal and signal ground. Would I notice an improvement by adding a 47k resistor there? (sadly it would be of crap quality IF i could find one lying around)

[Joe Rasmussen]

Originally posted by elizard



Thank you Mr. Joe.

Mr. Joe ?

I've a couple more questions for you if you don't mind.

First off, I think I know the answer to this, but I'll ask to be sure. It doesn't matter if the gainclone chips run off of a lower voltage supply than the tube itself does it? (i've an IGC going now, and i would re-use its power supply with a couple additions for the vbigc)

Secondly, i understand the answer to use a 47k pot to ground. Should I be using this in my IGC as well? In my igc i basically left out the pot completely, so i guess there's no connection between signal and signal ground. Would I notice an improvement by adding a 47k resistor there? (sadly it would be of crap quality IF i could find one lying around)

Re PS:

Sorry, but it's not a good idea. You need better isolation. The Class AB of the output stage is significantly Class B (which strangely may account, to an extent, for the good sound quality that is so surprising about these chips). The Cathode Follower is referenced to the + power supply Z and any modulation of the output stage is a big No-No! Class B makes this a real danger. And I'm not even talking about potential LF instability as well (causing potential motor-boating).

You can get 30VA toroidals fairly cheaply these days, pick one that is 25V-0V-25V. Also connect the AC side of this tx to AC, by-passing power switch. This way the switch only turns on the Gainclone stage and if the tube stage is already on (allow 30 secs min) then no rude noises when powering up.

Re original IGC, I don't think Thorsten would mind me saying that I was the one pointing out the potential stability problem when gain drops below 20dB as advised by NS. In your case, IF the IGC was connected to the Lo-Z output impedance of the preamp, no problem. But IF it got disconnected the gain would drop to 0dB, and that could be bad, especially if it was connected to speakers. Potential violent oscillations is not a good diet for speakers!

So a resistor to ground is really insurance, but not likely to improve sound. The question is what value? Whatever it is, it is seen as being in parallel with the first 'feedback resistor.' It should also be before the cap. If your main feedback resistor is 220K and the input resistor is 10K, then additional 10K to ground will give you 'noise gain' of 11 which will be safe (min 10), but NOW your input Z will halve to 5K. Can your preamp deal with that? If it's a tube preamp it may also lead to excessive LF roll-off. So you can see the problem. One answer may be to DOUBLE all the values. Now you will get 10K input Z and same gain structure.

Hope that clears rather than confuses things.

Joe R.

[moamps]

Originally posted by Nuuk
If you use different sets of pins on each valve, you can swap the valves over later to extend their lives!

Hi,

I'm not sure that valves live is extended so much in this application. Heater of both triode is always on, cathode of unused triode will anyway generate free electrons, those electrons will slowly move to another anode (noise?), and so on. Also, I can suppose that cathode of unused triode has higher (dangerous?) temperature because pins are unconnected and current don't flow (thermal energy transition). Maybe someone has experience about this (dhaen?).

Regards

[elizard]

Joe:

Thank you for answering both questions. Now, first one I don't think I made myself clear, so I'll repeat it later, but I'll address the second one now.

I understand your clarification, but two comments.
One - I don't have a lot of the necessary knowledge yet (its coming slowly!), but when you say that the pre-amp will have to deal with 5K, i've no idea whether it will be able to or not. Its a solid-state pre-amp kit i built http://velleman.be/common/product.Aspx?id=342550 (that's the link for it)
if someone can tell me whether it can or not, that'd be great!
also, when you say double all the values, you mean i should use 440k/20k for the other 2 resistors?

(and i know what you mean about the input being disconnected while the speakers are connected .. unfortunately i had the joy of experiencing that one two nights ago while i was playing around. needless to say, the amp was shut down quite quickly by me until i re-connected everything .. *whew* no damage!)

now, for the first question about power supplies
I'll try to make it clearer.
Right now I have a 400VA toroid, that's 2x22VAC secondaries. That's what I'm using for my IGC at the moment, and sounds great.
If I was to build the VBIGC, what I was wondering is if I can use that toroid in the PS, along with one 30VA 2X25VAC and one 5-10VA 2x9VAC (i think it was 9 volts out). What my concern is, is that the toroid i have now is 22VAC secondaries. Not sure if that's sufficient or not (all my knowledge suggests it should be just fine, but i like to double check anyway)

I read somewhere (and already guessed it) that sharing the toroid between the chips and the valve is a no-no, so that i didn't even count on. And as you said, its a very small price to pay for a 30-50VA toroid these days, so i'm not too worried about that. I'd just like to be able to use my old one as well, don't want to pay ~$100 for it again (after shipping)!

Thank You

edit: one other question while on topic .. what about using EI transformers (from hammond)? OR if someone living in canada/US can suggest a good place that I can get a toroid from for not too much? (Plitron is around $60 for a 30VA, $63 for 50VA!)

[Joe Rasmussen]

Originally posted by elizard
Joe:

when you say double all the values, you mean i should use 440k/20k for the other 2 resistors?

That's right. Then add another 20K to ground before the cap. So the input circuit will be, first 20K to ground, the input coupling cap 2.2uF minimum then a second 20K to the "-" input on chip and 440K (470K will do also) between "-" and main output. This will solve the 20dB instability problem and also prevent it from going into to 'unity gain" (that is 0dB gain) when accidentally detached from preamp.

(and i know what you mean about the input being disconnected while the speakers are connected .. unfortunately i had the joy of experiencing that one two nights ago while i was playing around. needless to say, the amp was shut down quite quickly by me until i re-connected everything .. *whew* no damage!)

Very nasty!

Right now I have a 400VA toroid, that's 2x22VAC secondaries. That's what I'm using for my IGC at the moment, and sounds great.
If I was to build the VBIGC, what I was wondering is if I can use that toroid in the PS, along with one 30VA 2X25VAC and one 5-10VA 2x9VAC (i think it was 9 volts out). What my concern is, is that the toroid i have now is 22VAC secondaries.

Don't see any problems there. Using the 22VAC x 2 400VA just means you will have the same power output you have now, so go right ahead.

what about using EI transformers

For the 30VA? Why not. It seems you have things well in hand. I think that 30VA txs from the sources you mention are a bit expensive, I can get 30VA toroidals for about a third of those prices.

Joe R.

[elizard]

Thank you once again for the answers, I appreciate them greatly, and they clarify everything

Where, if I may ask, would one get these 30VA toroids for 1/3 of the price I stated?
Even the EI transformers would cost me $30ish

wait, you're in AU aren't you? the shipping would eat up any savings i might have

if you, or someone else, do know a source in canada though, that'd be the best solution!

[Ropie]

Hi

I've just built a VBIGC from the schematic of the JR original. However, I am getting a rapidly fluctuating dc offset reading between 40mV and 135 mV on both channels. (I have built a gc before but valves are new to me.)

Does anyone have any initial ideas about what may be causing this?

thanks..