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 Post subject: VRM question
PostPosted: Wed Mar 25, 2015 9:34 am 
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Before I place my tweed order, I wanted to know if the VRM can be completely removed from the circuit if so desired. I like the idea of the VRM, but want to know if I can play the amp and completely bypass the VRM -- or -- is the VRM always involved, even if adusted to maximum volume. I hope my question makes sense. I did some searching in the forum and could not find a similar question, I apologize if this has been previously addressed though.

Thanks,
Bigfoot


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 Post subject: Re: VRM question
PostPosted: Wed Mar 25, 2015 10:46 am 
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When the VRM is on max, there is a 5 volt voltage drop, so it's basically out of circuit electrically.

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 Post subject: Re: VRM question
PostPosted: Sun Aug 30, 2015 5:30 am 
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I have B+ at around 395v and I'm trying to get it down a bit.

If you adjust the VRM pot to set the B+ to a lower voltage (350v) do you retain the sag aspect of the amp or is using a zener diode on the CT to reduce the B+ a better method?

Cheers.


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 Post subject: Re: VRM question
PostPosted: Sun Aug 30, 2015 12:37 pm 
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Either way, sag remains since sag is due to the tube rectifier. If you have a VRM, just dial it down, or alternately, use an old stock (or JJ) 5E3.

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 Post subject: Re: VRM question
PostPosted: Sun Aug 30, 2015 5:54 pm 
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What would you expect the voltage difference to be with a NOS GE 5Y3 rec to get sag?

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 Post subject: Re: VRM question
PostPosted: Sun Aug 30, 2015 6:04 pm 
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I would expect a 30-50 volt B+ drop from a new Sovtek, for example. Sag wise, not sure what. it would be.

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 Post subject: Re: VRM question
PostPosted: Sun Aug 30, 2015 6:11 pm 
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I'm getting about a 20v drop with a NOS GE 5Y3 rec. Could be why I can't get any sag.

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 Post subject: Re: VRM question
PostPosted: Sun Aug 30, 2015 10:57 pm 
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Sag refers to the drooping of the power supply voltage in response to large transient signals, which lends a certain dynamic "feel" to the tube amplifier that is not generally found in solid-state amplifiers.

Sag is generated because of the internal resistance of the rectifier tube. A tube rectifier exhibits a fair amount of voltage drop which varies with the amount of current passing through the tube. In a class AB amplifier, the current drawn from the power supply is much greater at full power output than it is at idle. This large change in current demand causes the voltage drop across the tube rectifier to increase, which lowers the available plate supply voltage to the output tubes. This lowering of the supply voltage lowers the output power slightly in opposition to the larger input signal, making it act like a compressor. The lowered supply voltage also tends to decrease the available headroom, increasing clipping and changing the operating point of the tube dynamically.

All that to say, the voltage drop at idle is not an indication of sag. It is the dynamic voltage drop that causes sag. You would have to measure the voltage while you player the amp to see how much sag there is. But you should be able to hear it when you play.

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 Post subject: Re: VRM question
PostPosted: Mon Aug 31, 2015 2:38 am 
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That's exactly what I did. I connected a dmm to ground and to the first filter cap and then played watching the dmm.

The biggest drop I got was around 20v - 25v.

I do have the VRM kit fitted. I also installed the "Percy" vol/ tone mod. Would the latter have any impact on reducing voltage drop?

If it's relevant I put a TAD 5Y3 rec in and the voltage went up to 420 volts but I still only got a 20v drop.

I have upgrade the PT to MM Toneclone FTDP/240-M (380v - 0 - 380v) and the OT to MM Toneclone FTDO-59.

There is a 270ohm bias resistor.

Cheers.


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 Post subject: Re: VRM question
PostPosted: Mon Aug 31, 2015 9:27 am 
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Recommend you pull the TAD 5Y3. Or permanently turn down VRM. Voltage is too high. Was this a Trinity Kit? If so, Put back the JJ 5Y3.
But You are getting 20V-25V sag.
No, the mods will not have any impact on voltage.

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 Post subject: Re: VRM question
PostPosted: Mon Aug 31, 2015 10:12 am 
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The VRM kit is yours, the 5E3 kit is from TAD which I understand to be a Mojo kit with TAD valves and trannies.

The TAD 5Y3 was already out! I just put it back for comparison.

From what I've read the GE 5Y3 should drop around 50v. Don't know if that would make any material diffence to a saggy sound over a 20v drop. The amp has never sounded saggy to me and is something I have been chasing for a while. Guess it's just not meant to be.

Thanks for your feed back, really appreciate it.

Cheers.


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 Post subject: Re: VRM question
PostPosted: Mon Aug 31, 2015 10:15 am 
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You CAN increase sag, BUT you need to add a 100R to 200R 10 watt power resistor directly to the B+ line.

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 Post subject: Re: VRM question
PostPosted: Mon Aug 31, 2015 10:28 am 
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OK, I'll give that a go.

Whereabouts would the resistor go in relation to the VRM kit?

Cheers.


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 Post subject: Re: VRM question
PostPosted: Mon Aug 31, 2015 10:41 am 
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Assuming you are scaling the Whole Amp, after the B+ Out Line. i.e. before connecting to the board/OT

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 Post subject: Re: VRM question
PostPosted: Mon Aug 31, 2015 12:27 pm 
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OK thanks.

I've ordered a couple of 100r 10watt power resistors.

Thanks again.

Cheers.


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 Post subject: Re: VRM question
PostPosted: Mon Aug 31, 2015 8:30 pm 
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mayburyds wrote:
I've ordered a couple of 100r 10watt power resistors.


Use one, and if you want more sag, add the second one.

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 Post subject: Re: VRM question
PostPosted: Thu Sep 03, 2015 3:49 pm 
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Hi, started off with one resistor but didn't hear or measure any change. Put the second one in and still no change. I also seem to have lost the 20v drop I was getting before so I'm beginning to think I have an underlying problem with the PT connections and possibly the grounding scheme.

Cheers.


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 Post subject: Re: VRM question
PostPosted: Fri Sep 04, 2015 8:08 pm 
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A standard technique to add sag to a solid state rectifier is to add resistors so they may be an other issue, but cant guess what it is.

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 Post subject: Re: VRM question
PostPosted: Sat Sep 05, 2015 5:03 am 
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I think I have taken up to much of your valuable time now.

I really have appreciated your help, especially as I have only purchased the VRM kit from yourself and not a complete 5E3 kit.

Many thanks.


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 Post subject: Re: VRM question
PostPosted: Sat Sep 05, 2015 7:39 am 
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No problem.

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