Tube Amps—The Heat

Tube Amps—The Heat

Obviously tubes generate the most heat in an amp.

I have a learned British friend who is fond of saying how good tube amps sound after they warm up—giving all credit to the tubes.

But somewhere on the internet I read that it's the resistors that really start to cook and make that warm tone warm.

My Thunder I Reverb has 3 big resistors under the hood and one on the reverb tank. I haven't owned the amp long enough, or had it working long enough before dismantling it, but wonder if you guys in the know have checked the thermal effect of these components and how you feel they effect tone.

Guild chose to drill the chassis (and leave the chassis ends open) for improved cooling.

What do you think?

Re: Tube Amps—The Heat

Archer993 said:

I have a learned British friend who is fond of saying how good tube amps sound after they warm up—giving all credit to the tubes ... But somewhere on the internet I read that it's the resistors that really start to cook and make that warm tone warm.

My Thunder I Reverb has 3 big resistors under the hood and one on the reverb tank ... but wonder if you guys in the know have checked the thermal effect of these components and how you feel they effect tone.

Click to expand...

Big resistors first; the power supply of your amp and every amp has to provide a number of different voltages all from the same source ... the rectifier. It does so by using resistors - the red arrows - in between the connection points to reduce the power to the required point at the next tap.

In the pic below, notice the supply voltages indicated by the green arrow are lined left to right up in decreasing value. You can check your schematic but 1. is the center tap / primary for the 6GW8 plates 2. is the phase inverter 3. is the tremolo oscillator and so on. Each of the these values is achieved by inserting a resistor of the value/ohms necessary to drop the voltage to the value required at the next tap:




The big resistors you mentioned ... two of them - magenta circles - are shown below (Zulu's T1 in the pic) and the third - green circle - is down at the other end of the amp at the reverb can cap.




Your big resistors are in the power supply; the musical signal never sees them and, other than not drifting in and out of tolerance, have nothing to do with tone quality.

Tubes / warmth / resistors: tube amps are warm with respect to post-1960s solid state amps. Some tube amps are warmer than others; in part by design and in part by the interaction of all the large parts ... power and output transformers and small parts ... capacitors and resistors that make them up.

Re: Tube Amps—The Heat

Heat isn't a factor imho. Tubes get hot, the chassis heats up, not a problem. Usually when amps get searingly hot to the touch, it's because the chassi used is thin/cheap. Some of the original Vox AC50's and AC100's used to burst into flames due to insufficient venting, but I don't think you have anything to worry about. One of the byproducts of those big power resistors dropping voltage is heat, btw.

Re: Tube Amps—The Heat

Default said:

Heat isn't a factor imho. Tubes get hot, the chassis heats up, not a problem. Usually when amps get searingly hot to the touch, it's because the chassi used is thin/cheap. Some of the original Vox AC50's and AC100's used to burst into flames due to insufficient venting, but I don't think you have anything to worry about. One of the byproducts of those big power resistors dropping voltage is heat, btw.

Click to expand...

I realize the 3 power resistors under the chassis are not in the tone circuit and are a heat source - possibly the most significant heat source of the components below. I seem to remember reading somewhere that heat and resistance go hand in hand. I also know that heat can lead to component failure as everything has a hi and low temperature operational range.

I just wonder what the affect on all the other resistors and caps would be if these heat sources were moved out of the chassis. It's academic . . .

Re: Tube Amps—The Heat

Hi Archer; I apologize for the mis-read; I thought you were talking about 'warmth' in the tone sense as opposed to 'heat'.

Re: Tube Amps—The Heat

capnjuan said:

Hi Archer; I apologize for the mis-read; I thought you were talking about 'warmth' in the tone sense as opposed to 'heat'.

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No need . . . . I didn't take it wrong.
There is great value for hackers like myself in your explanation of the posted schematic because it shows a logical analysis of how to determine how critical a component is to the tone pathways—got me to appreciate the value of a clear schematic and that I should be looking harder at the schematic before picking up a soldering iron or mousing orders for parts I may not need.

Getting back to heat - if the amp is being played at 10 I assume max current is flowing through all components. The chassis is doing its job as a heat sink for the trans and tubes and the resistors are limiting flow. That must be the time when the amp would reach it's highest operating temp. So if the 3 power resistors have a max cumulative rating of 27 watts (which probably never happens) they could be spilling 5?, 10?, 15? watts under the chassis, which to me is a lot of heat for a tight space.

All right, enough about the heat. I'm obsessed. Probably because it just snowed another 4 inches here . . . .

Re: Tube Amps—The Heat

I have a clean T1 schematic in a PowerPoint file; I'm getting these readable images by enlarging the size of the view in PP and copying them off with Gadwin's ever-so-helpful screen-capture utility that default (posing as a guy named default) showed us. Anyway, that's how I can get legible images. Yes; heat is the enemy of electronics.

Don't know about your version 2 but the early models had a heat shield stapled to the underside of the top to abate the risk of catching the cabinet on fire ... although not every idea in amp-building is a good one. When taking the chassis out of the version 1 T1s, the heat shield - industrial thickness aluminum foil - almost always snags and bends to the point where it gets in the way of putting the chassis back in. "Not every shot lands on the green" (Tiger Woods; famous golfer and amp-builder) Nevertheless, the version 2s are better-made amps than the version 1s.

On the subject of heat, everybody here (who lives up north) has gotten annoyed with my rubbing it in ... so ... no reason why you should feel any different :wink: ... right now, 71 degs, winds light and variable, not a cloud in the sky 8)

Re: Tube Amps—The Heat

The first amp I overhauled was a Supro Thunderbolt. It has the aluminum foil layer which has a ground wire attached to it.

I Just finished doing over a Silvertone 1483 with partial shielding under the amp section and full shielding under the control section.

My understanding was that the foil was for rf shielding, but it makes sense as a heat barrier too.

Re: Tube Amps—The Heat

Archer993 said:

.. My understanding was that the foil was for rf shielding, but it makes sense as a heat barrier too.

Click to expand...

Ok; rf shielding hadn't occurred to me; does you amp have the shield? The early version has it however if the foil is the least bit wrinkled from all the heat, the upper edge of the faceplate snags it as the chassis is tugged out the back.

It can't be flattened from the front through the gap between the faceplate and the top and it can't be flattened from the back because there's too little clearance between the chassis and the top.

I like features as much as the next guy but I especially like the ones that don't self-destruct. :evil: :wink:

Re: Tube Amps—The Heat

Yes this amp has the foil, and it's in good shape.

I replaced the aluminum foil on the Silvertone with some copper foil I had. The amp is dead quiet - pretty good for a relic from 1961.