Educate me on plate voltage please!

A

azzzy

New member
I would like to understand the practical implications of plate voltage. Let's say the amp's plate voltage is rated at 450 volts. Is it constant? Or does it vary with the output? Is it 450 volts only when the amp is cranked to the max or is it always 450 volts even with the volume turned down all the way?

What happens when the amp's plate voltage exceeds the tubes' rated plate voltage? Let's say I plug in a tube rated up to 400 volts into an amp that has 450 volts plate voltage?

Is there a way to mitigate plate voltage via tube bias?
 
Laymen's version as I don't get into all the geek speak on most things. It's not a constant if there are tubes installed. It will also fluctuate with the wall voltage. The tubes pull current in relation to the bias which will lower the plate voltage. It should vary with output but only in relation to the tubes current draw increasing as you turn the amp louder.

Tubes are rated but most can handle more. The older tubes were generally rated higher. I wouldn't really be concerned unless your amp is putting out well over 500 volts. Higher plate voltage may make your amp brighter, punchier, less sag. I like it!

There are some transformers that have high and low plate voltage taps that can be put on a switch or just wired for one. There are other ways to lower your voltage. It will lower the bias usually too much unless you have it as a constant and can rebias for that specific voltage. It can be tricky to do but it can be done. I have seen it done on a switch where the bias also lowered when selected. Paul Cochrane had this option on his Colonial amp if I remember correctly.
 
glip22":37qeb33p said:
Laymen's version as I don't get into all the geek speak on most things. It's not a constant if there are tubes installed. It will also fluctuate with the wall voltage. The tubes pull current in relation to the bias which will lower the plate voltage. It should vary with output but only in relation to the tubes current draw increasing as you turn the amp louder.
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First, thank you for replying. Now, so what you're saying is if I keep the main output at say half way up there is less chance of blowing the tubes by excessive plate voltage? Am I understanding this correctly? Any ballpark idea of how much less plate voltage there would be with the volume half way up as a percentage of the max plate voltage with the volume up all the way?

glip22":37qeb33p said:
Tubes are rated but most can handle more. The older tubes were generally rated higher. I wouldn't really be concerned unless your amp is putting out well over 500 volts. Higher plate voltage may make your amp brighter, punchier, less sag. I like it!
Click to expand...

I have 2 6L6GC amps left - a Fender HRD (allegedly 430 plate volts) and a Boogie Trem-O-Verb (depending on the settings between 415 and 530 plate volts). I want to try my vintage Mullard EL37's in these amps. EL37's are allegedly rated up to 400 plate volts (although according to some sources up to 800 v). I suppose the Fender with its 430 v should be safe for EL37's? Especially since I never turn it up past 10 o'clock anyway. What kind of damage could occur if things don't go as planned? Is there a danger of damaging the transformer?

As for the Trem-O-Verb the tube rectifier mode supposedly drops the plate voltage to 415 v, which I guess should not be an issue for EL37's, right? Even in the bold setting?

I realize these are all hypothetical - just would like your personal opinion.

glip22":37qeb33p said:
There are some transformers that have high and low plate voltage taps that can be put on a switch or just wired for one. There are other ways to lower your voltage. It will lower the bias usually too much unless you have it as a constant and can rebias for that specific voltage. It can be tricky to do but it can be done. I have seen it done on a switch where the bias also lowered when selected. Paul Cochrane had this option on his Colonial amp if I remember correctly.
Click to expand...

I'm not looking to mod my amps - just want to try these cool looking tubes in them. :LOL: :LOL:
 
With a NMV amp I tend to like a lower plate voltage for lead playing. Higher for rhythm playing. It has as much effect on the feel as the sound.
 
azzzy":e3gqbk95 said:
glip22":e3gqbk95 said:
Laymen's version as I don't get into all the geek speak on most things. It's not a constant if there are tubes installed. It will also fluctuate with the wall voltage. The tubes pull current in relation to the bias which will lower the plate voltage. It should vary with output but only in relation to the tubes current draw increasing as you turn the amp louder.
Click to expand...

First, thank you for replying. Now, so what you're saying is if I keep the main output at say half way up there is less chance of blowing the tubes by excessive plate voltage? Am I understanding this correctly? Any ballpark idea of how much less plate voltage there would be with the volume half way up as a percentage of the max plate voltage with the volume up all the way?

glip22":e3gqbk95 said:
Tubes are rated but most can handle more. The older tubes were generally rated higher. I wouldn't really be concerned unless your amp is putting out well over 500 volts. Higher plate voltage may make your amp brighter, punchier, less sag. I like it!
Click to expand...

I have 2 6L6GC amps left - a Fender HRD (allegedly 430 plate volts) and a Boogie Trem-O-Verb (depending on the settings between 415 and 530 plate volts). I want to try my vintage Mullard EL37's in these amps. EL37's are allegedly rated up to 400 plate volts (although according to some sources up to 800 v). I suppose the Fender with its 430 v should be safe for EL37's? Especially since I never turn it up past 10 o'clock anyway. What kind of damage could occur if things don't go as planned? Is there a danger of damaging the transformer?

As for the Trem-O-Verb the tube rectifier mode supposedly drops the plate voltage to 415 v, which I guess should not be an issue for EL37's, right? Even in the bold setting?

I realize these are all hypothetical - just would like your personal opinion.

glip22":e3gqbk95 said:
There are some transformers that have high and low plate voltage taps that can be put on a switch or just wired for one. There are other ways to lower your voltage. It will lower the bias usually too much unless you have it as a constant and can rebias for that specific voltage. It can be tricky to do but it can be done. I have seen it done on a switch where the bias also lowered when selected. Paul Cochrane had this option on his Colonial amp if I remember correctly.
Click to expand...

I'm not looking to mod my amps - just want to try these cool looking tubes in them. :LOL: :LOL:
Click to expand...

When it comes to a tube failing it's not so much your plate voltage you need to worry about. It's your screen voltage. I wouldn't get too caught up in this. Those Mullards will be fine at your lower plate voltages granting you have adjustable bias in your amps, and can bias them in range. Do you know how to bias? I am not sure if the Boogie is fixed bias? If it is the tubes will probably not be plug an play.
 
glip22":je0pweui said:
When it comes to a tube failing it's not so much your plate voltage you need to worry about. It's your screen voltage.
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What should I worry about with screen voltage? What should I pay attention to? Are there symptoms I should watch out for and if they are there is there a time to react (turn off the amp) before any damage occurs?

glip22":je0pweui said:
Those Mullards will be fine at your lower plate voltages granting you have adjustable bias in your amps, and can bias them in range. Do you know how to bias?
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Only as far as plugging in a Biasrite and turning the bias trim pot. :) I usually look up the recommended mA's and bias the tubes to that. I have never really had to bias the tubes for the plate voltage.

glip22":je0pweui said:
I am not sure if the Boogie is fixed bias? If it is the tubes will probably not be plug an play.
Click to expand...

Yes, the Boogie is a fixed bias amp. But the EL37's are a direct replacement for the 6L6's and their derivatives, so they should be biased in the same range. If the Trem-O-Verb plate voltage in the tube rectifier mode is at 415 v that would actually make it lower than the Fender. Although frankly I'm not overly concerned with the Boogie - I haven't touched it in years - it's been sitting in my storage nook all this time collecting dust (I should probably put a cover on it or something :D ). Ultimately I'd be curious to try EL37's in a Soldano SLO but I don't own one at the moment. And the SLO I think has 500 v plate voltage (if not higher). I guess Boogie would be a good test whether it's even worth having EL37's in a high gain amp to begin with.
 
azzzy":2cggk0ad said:
I would like to understand the practical implications of plate voltage. Let's say the amp's plate voltage is rated at 450 volts. Is it constant? Or does it vary with the output? Is it 450 volts only when the amp is cranked to the max or is it always 450 volts even with the volume turned down all the way?
Click to expand...

The plate voltage comes from the Power supply. For a push/pull amplifier (which accounts for most guitar amps) the voltage is supplied to the center tap of the primary winding, the outer taps of the primary connect directly to the tube plates. One to the left bank, and the other to the right bank.

So basically if your plate voltage is 450vdc, it stays pretty much 450vdc. However it can sag lower depending on the power supply, and volume. Only time it will go higher is if you have too much voltage coming in from the wall AC. Also some people remove 2 outer power tubes in a 100w, this will cause the plate voltage to go higher since part of the load has been removed.

azzzy":2cggk0ad said:
What happens when the amp's plate voltage exceeds the tubes' rated plate voltage? Let's say I plug in a tube rated up to 400 volts into an amp that has 450 volts plate voltage?
Click to expand...

The life of the tube is shortened. However this is not always true, Some tubes can handle well over their rated plate voltage.

azzzy":2cggk0ad said:
Is there a way to mitigate plate voltage via tube bias?
Click to expand...

No
 
Before you put a EL37 in the Mesa amps I would highly recommend asking Mesa about it.
If you really want a blend of some of the EL34 and 6L6 you might want to check out a 6CA7.
 
One thing I just discovered is if your B+ is like 480-490 or even 500v or higher, it's not the tubes you should be worrying about. It's the filter caps, which are typically only rated for 500v.
 
stephen sawall":h85wi72u said:
Before you put a EL37 in the Mesa amps I would highly recommend asking Mesa about it.
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Meh. What are they going to say other then "use only Mesa/Boogie tubes"? :dunno:

stephen sawall":h85wi72u said:
If you really want a blend of some of the EL34 and 6L6 you might want to check out a 6CA7.
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That's not the reason. I just like the Mullard (well, the Philips) sound. Been using Mullard/Amperex tubes in all my EL34 (and EL84) amps for over a decade. Granted I have no idea what EL37's sound like but that's the reason I want to try them. Basically the idea is to see what a Mullard tube would do to a 6L6GC amp. :rock: In terms of sound, not in terms of physical damage. :D
 
FourT6and2":2c8v0z6p said:
One thing I just discovered is if your B+ is like 480-490 or even 500v or higher, it's not the tubes you should be worrying about. It's the filter caps, which are typically only rated for 500v.
Click to expand...

I suppose if the filter caps are OK with 6L6GC's they should be OK with EL37's, no?
 
azzzy":16612r5h said:
FourT6and2":16612r5h said:
One thing I just discovered is if your B+ is like 480-490 or even 500v or higher, it's not the tubes you should be worrying about. It's the filter caps, which are typically only rated for 500v.
Click to expand...

I suppose if the filter caps are OK with 6L6GC's they should be OK with EL37's, no?
Click to expand...

It's the B+ you need to worry about, not the type of tube, when it comes to something like that. The caps in my amp were getting physically warm with 490v on the plates. Had to switch to low voltage taps on the PT. Shouldn't be an issue in your situation. Just thought I'd mention it.
 
stephen sawall":1aaewqos said:
Before you put a EL37 in the Mesa amps I would highly recommend asking Mesa about it.
If you really want a blend of some of the EL34 and 6L6 you might want to check out a 6CA7.
Click to expand...

Kt77s also seem to have a good blend of EL34 and 6L6. Mids of a 34 and bottom end of a 6L6. :thumbsup:
 
phil b":1y5hz8hj said:
Kt77s also seem to have a good blend of EL34 and 6L6. Mids of a 34 and bottom end of a 6L6. :thumbsup:
Click to expand...

I am currently running GEC KT77's in all my EL34 amps (in place of my beloved Mullard xf1's and Amperex xf2's). The idea here is to try Mullard direct replacement for 6L6's.
 
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