50/60 Hz transformer question

[Mick Bailey]

I've just been checking over an amp and it's showing lower than expected voltages. It's a US-only model, 120v/60Hz. I'm running it off 120v/50Hz for test purposes. Usually it's run off a 240v/120v auto-transformer

The EL34 heaters are reading 5.48Vac and preamps are 9.7Vdc. B+ is 440V.

I noticed the PT growls a little more than usual. My though is that the PT may be tightly specified for 60Hz. Could it be saturating and losing efficiency to the degree of dropping the output voltage on 50Hz?

[pdf64]

How much current is it pulling, especially at full load?
I think that if the mag circuit saturates, the primary inductance will collapse, resulting in a big increase in mains current.
I doubt that the mag circuit will saturate at low loads, eg idle, standby.

[Mick Bailey]

It pulls 460mA at idle and 2.58A at full output at the point of clipping. I've just been searching for info on the amp (a Guytron GT-100) and found others have measured low heater voltages even in the US. The B+ seems a little low as well, though.

The amp is perfectly happy and the PT doesn't get unduly hot. The growl was due to the transformer vibrating the steel chassis. There are some disproportionately heavy/large clamping washers holding the transformer down, suggesting there may have been a need to improve the mounting at some stage.

It's an unusual design - the preamp drives a 2xEL84 power amp, which in turn drives the main 4xEL34power amp.

[pdf64]

It pulls 460mA at idle and 2.58A at full output at the point of clipping. I've just been searching for info on the amp (a Guytron GT-100) and found others have measured low heater voltages even in the US. ..the preamp drives a 2xEL84 power amp, which in turn drives the main 4xEL34power amp.

All seems ok then?
What an unusual design - schematic?
Do you think it's intended / able to achieve AB2?

[Mick Bailey]

Turns out it does look OK, though I don't have the means to A-B it on 50/60Hz. Perhaps there would be a slight efficiency improvement at 60Hz, though it would depend on the lamination material.

I couldn't find a definitive schematic, though the patent shows and describes the main design elements. It isn't intended to achieve AB2, though. It could have been a more interesting amp if it had been.

Patent US6111961 - Amplifier for a guitar or other musical instrument - Google Patents

[Enzo]

If you are not using the step down, where are you getting the 120v? And is it holding at that voltage? Just making sure.

[Mick Bailey]

It's off my bench variac - the owner wanted to hang on to his auto transformer. I set it to 120v and then double-checked the voltage at the amp mains switch. It's a 2400VA variac and holds its voltage under load, but I re-checked just to be sure. Plus - the usual trap - checked my meter battery.

[g1]

Got a big power amp you can generate 60hz with?

[Tom Phillips]

Got a big power amp you can generate 60hz with?

Interesting that you mention that. I was given a pair of Bogen Model MT250 power amps. They are solid state amps with multiple tap output transformers. One of the taps will provide up to 300W of 120 V sine wave from 35 to 10,000 Hz when the amp's input is driven from a suitable source. I have never used them to do that but the possibility is Intriguing.

[Mick Bailey]

Got a big power amp you can generate 60hz with?

Now there's a bit of lateral thinking. I'll take a look what I have - I've certainly got some 2KW+ amps with one working channel to check out.

I realized I'd transposed two digits on the AC reading - it's actually 5.84V, so not as low as I'd originally though and not too far out, though still on the low side. I ran the amp for a couple of hours to get everything warmed up and to check for issues. Everything seems to be fine - no overheating or voltage drift and good, loud output - plenty of clean headroom, but an unusual clipping characteristics, perhaps due to it having a 15W amp effectively re-amped through the main power section. The driven sound can get pretty fizzy as you'd perhaps expect, given that the drive characteristics are that of a pair of EL84s driven hard.

[J M Fahey]

You may have other problems but 50Hz is not one of them.

I live in Argentina, a 50 Hz Country, and Musicians have been buying thousands US amps (99% of them Fender) on their holiday trips to Miami, NY or LA, for the last 50 years at least , a customer regularly brings his cream or off white Fender Bassman 50 "piggyback" , complete with clamp downs to hold head fixed to cabinet and tilt back legs,

they are all used with 220/110 transformers and all perform flawlessly.

As of the Guytron: it was a *revolutionary* amp, the first offering "reamping" , in this case a small 10/15W tube amp driving a resistive load and then reamped, in this case by a real tube amp.

Killer idea, but far too advanced at the time it was introduced, remember people in general do not buy things because they are good, but because of publicity or reading about them in Forums or because "Jimi used it" , none of which applies here.

To boot it was heavy and expensive ... if you forget it was also unique.

+V 440V is fine, if you think this is an amp designed specifically to attenuate way too high stage levels, in a controlled way , no need to push for maximum possible volume.

Now the lower than normal filament voltage does not look that good, but it seems to be a factory decision, so .....

[Mick Bailey]

I've searched quite a bit about these amps. There isn't much information, but one thing that turns up is a few references to the low DC heater voltage in the preamp stages. I recall a letter to "Practical Wireless" in the early 1950s where Osram had suggested to reduce noise in sensitive preamp stages the heater voltage could be substantially reduced. I've just been looking through my pile of mags but can't locate it. Frustrating - it's in there somewhere. Maybe the same reasoning is applied here to keep the noise down. As it stands it's a really quiet amp and perhaps every technique needs to be used to reduce noise getting carried over into the main power amp.

It's a very different sounding amp to anything I've come across before and as you say, heavy. Four transformers at one end. Even the 15W output transformer is way larger than you'd find in most amps. It gets really close to a whole range of classic amp sounds and could probably considered as a true analogue modelling amp.

There are precious few really different amps I can think of. Stepping outside of the FMV triangle can be risky for manufacturers. Despite the technology out there, amps are still referenced to established designs. One spectacular amp is Laney's SE 50W class A head. 5xEL34 in parallel. Incredible sounding, tone to die for, but not popular.

[Dave H]

I've searched quite a bit about these amps. There isn't much information, but one thing that turns up is a few references to the low DC heater voltage in the preamp stages. I recall a letter to "Practical Wireless" in the early 1950s where Osram had suggested to reduce noise in sensitive preamp stages the heater voltage could be substantially reduced.

There's talk of using 5.5V - here

I know a 12AX7 will operate at 3.15V because I once wired the heaters of the first tube in series by mistake. It ran like that for years before I noticed. I always wondered why that amp was slow to warm up.

[Mick Bailey]

I built a copy of a relatively early tube amp that was intended as an add-on to a crystal set, which I also built (and still have). The volume was controlled by reducing the heater voltage. One feature of it was the noise would drop before the volume drop became noticeable. The other remarkable thing was just how little heater voltage was needed to give reasonable output.

An interesting experiment would be to connect up the heaters of guitar amp via a 6v transformer and variac and observe how the amp sounds under low heater voltage. Keep the power tubes and PI at full voltage though.

[pdf64]

One spectacular amp is Laney's SE 50W class A head. 5xEL34 in parallel. Incredible sounding, tone to die for, but not popular.

Wow, it only weighs 21kg L50H | LIONHEART | Laney
Even if it's a SMPS (?), how do they keep the weight of the OT down?