Estey T12 guitar amp

[Chuck H]

The probability of sag compression and/or ripple harmonics being detectable and desireable by the player is really cool. I remember early on in my learning, back when brick and mortar electronics stores still existed, having a conversation with an old timer that liked to build guitar amps. He told me that once you have designed and built something you like you should never change it in any way. And mentioned that he had amps prototyped on bread boards where the actual test board was simply stuck into the amp for fear of losing any mojo from the layout if he rebuilt it. This started me on the path to understanding those less obvious things about amp design that can affect the overall sound. Since much of this aspect can be somewhat arcane it puts a little of that "black art" or alchemy back into the genre. Which is fun. Sometimes we can recognise why something sounds good only after it's been done and reverse engineered. And these discoveries still happen. Since guitar amps are instruments unto themselves and not strictly tonal reference/reproduction tools it's the things that make them MISbehave in a musical way that we are interested in

[johnhoef]

Helmholtz,
I read this experiment, and it certainly zeros in on exactly what I am dealing with here. In summary, in order to make my guitar player customer happy, I will have to not only keep the tube rectifier, but also adjust the filter capacitor to re-introduce some ripple into the power supply! Another thought comes to mind.....installing a resistor between the 6X4 cathode, and the filter capacitor....in fact, I could even make that an adjustable resistor, and let my friend play with that.
Just a little explanation of my new circuit design. D1 and D2 serve two functions. They contribute to a solid state bridge circuit to supply DC to the small signal tube heaters. Their secondary function is to mimic the more typical power supply design using a center tapped transformer with a typical rectifier tube. Essentially a hybrid bridge. During the positive swing from the transformer, D2 conducts, essentially grounding the lower 6X4 plate as well as the lower wire of the transformer. Then, of course, the reverse happens. This design exposes the 6X4 to the same full wave as if a center tapped transformer was used. There is no hum with this design, and once I get the "sag" just right, I will have a happy customer!

[johnhoef]

Chuck,
Well said, concerning the "lo-fi" aspect of guitar amps! I spend most of my time in the hi-fi world, and a solid ripple free super quiet power supply is absolutely desirable there! And here we are in the guitar amp world, where this mentality is totally undesirable! Here I am, after putting together a nice hum free solid state power supply for my customer, trying to make it less perfect!! He and I have talked often about this power supply sag, and he keeps insisting a tube rectifier is essential in getting that sound he is looking for. I keep thinking this "sag", along with a certain degree of ripple can somehow be accomplished using solid state rectifiers.

[Helmholtz]

Just a little explanation of my new circuit design. D1 and D2 serve two functions. They contribute to a solid state bridge circuit to supply DC to the small signal tube heaters. Their secondary function is to mimic the more typical power supply design using a center tapped transformer with a typical rectifier tube. Essentially a hybrid bridge. During the positive swing from the transformer, D2 conducts, essentially grounding the lower 6X4 plate as well as the lower wire of the transformer. Then, of course, the reverse happens. This design exposes the 6X4 to the same full wave as if a center tapped transformer was used. There is no hum with this design, and once I get the "sag" just right, I will have a happy customer!

Yes, it's a nice idea. The diodes D1 and D2 are the shared halve of 2 fullbridge rectifiers.

The hybrid bridge is explained here: https://www.valvewizard.co.uk/bridge.html

Just wondering: Would feeding the heaters from the floating secondary AC introduce hum?
Of course one advantage of your solution is that one end of the heater supply is grounded.

[johnhoef]

Helmholtz,
Are you suggesting powering the two preamp tube heaters using the AC off the isolation transformer? That would eliminate the solid state bridge, and associated filter. I could simply use two diodes to build the hybrid bridge. The 6X4 will still get it's heater power from the separate 6.3 volt filament transformer. Wiring that in series with the other two heaters does not work. The 6X4 heater draws far more current than the other two.

[Helmholtz]

Helmholtz,
Are you suggesting powering the two preamp tube heaters using the AC off the isolation transformer? That would eliminate the solid state bridge, and associated filter. I could simply use two diodes to build the hybrid bridge. The 6X4 will still get it's heater power from the separate 6.3 volt filament transformer.

Yes, but just wondering.
Do you filter the heater supply? Is it required?

[johnhoef]

Helmholtz,
I see I forgot to show the filter in the DC heater supply schematic...yes, I have a 50mfd filter in there.
Indeed, now that I have the hum problem solved, I can try supplying all heaters with AC. I know many audiophile grade tube amps do use DC to power the heaters, so I did the same in this case to get as much hum out as possible.
I'll try your suggestion........

[johnhoef]

I made a few more changes. I reduced the tube rectified filter back down to 50mfd. This is how it was originally with the half wave tube rectifier. I also disconnected the filter from the heater supply, feeding the raw DC to the preamp tube heaters. Got a noticeable hum! Then installed a 50mfd cap as a filter, and that eliminated the hum. I'll be returning the amp to the guitar player later today. Very curious to see what he thinks. I have a suspicion I will be doing some more fine tuning to this amp.

[johnhoef]

Thought I would add one final contribution to this thread. It's been a few months since I worked on this amp. Haven't heard beck from the guitar player owner. This is usually a good sign that he is satisfied. Thumbs up!
By the way, thank you all for your help...greatly appreciated!

[Chuck H]

He and I have talked often about this power supply sag, and he keeps insisting a tube rectifier is essential in getting that sound he is looking for.

Late to address this, but why not? Your customer is drinking the KoolAid of internet lore. Class A, single ended amps have virtually no sag. Even when clipping. Any sag from using a rectifier tube would, for the most part, just be a reduction in the plate voltage. Which would remain static rather than dynamic with playing.

I keep thinking this "sag", along with a certain degree of ripple can somehow be accomplished using solid state rectifiers.

In amp designs that DO exhibit sag I've simulated rectifier tube performance by simply adding series resistance to the power supply immediately following the SS rectifier. But this would be strictly voltage sag. I can't say how similar just adding resistance would be to a rectifier tube regarding ripple because I've never taken side by side tests. Though I have to assume that since the same principals are in effect that it should be just as similar.

[johnhoef]

Chuck,
I sure hear you. Having done work on many a guitarist's amps, I often hear about this "sag"concept. After a while it's hard for me to argue with them. They are musicians, and I expect they hear things that common Joes don't. As you may have read above, concerning my psych test with my guitar player friend, he did notice a difference after I installed a solid state bridge rectifier in this amp.

[g1]

In most cases, going to solid-state rectifier will increase the supply voltages enough to be noticed by the player. So in this case, it would be the 'drop' of the voltages due to the tube rectifier, rather than the dynamic 'sag' that players usually talk about (which ChuckH noted above).
Maybe nit-picking with the semantics a bit, but I think it is an important distinction in the terminology and the two different effects.

[The Dude]

I'm still hesitant to believe that most of it isn't psychological. It would be interesting to do several blind tests and have the player chose which amp has which rectifier.

[johnhoef]

G1, Dude,
The tube rectifier had a series resistor. When I installed the solid state bridge, I modified the series resistor so that the tubes saw the exact same B+. I think it was a pretty good phych test, considering he had no knowledge that I installed a solid state rectifier. There may well have been a difference. He was used to the way the amp sounded, and I believe he really did notice a difference.
Unrelated to this project, I have been confronted with other cases where the psych was quite present. My audiophile customers sometimes spend large amounts of money on "upgrades" to their sound systems. Now, did they really hear a difference, or due to the money spent, did they hear a difference because they expected to?
I performed another test a while back. A customer asked me to replace the electrolytic caps in his speakers crossovers. I did one speaker, and then asked him to come over for an A/B test. I had the speakers marked on the back as to which had the new audiophile grade film capacitors. He thought the speaker with the old electrolytic caps sounded better! Blind tests don't lie!