The Impressive Vetruvio GM70 SET amp.

[FullRangeMan]

See this amp on Ebay this week and Iam impressed with the GM70 SET dual mono, it have a regular bulb lamp in the output stage.
As you can see on the image below, there is a big light source inside the chassis.
As Iam not a expert in electronics this looks good to me. What are your opinion??
vetruvioaudio.com

[FullRangeMan]

Price is 700dollars on Ebay Vetruvio GM70/30 For Sale | AudiogoN

[FullRangeMan]

SCHEMATIC: There is one or two 25W bulb lamp on the B+(1K Volts) supply. I thanks in advance any opinion about this circuit.

[kevinkr]

It's been done, it's kind of inefficient and the linearity is just ok at best.. LF signals will severely modulate the hot filament resistance resulting in significant distortion. I wonder what sort of plate load these bulbs present to the output tube, (maybe slightly less than 5K for a pair of 240V/25W bulbs) and the RC coupling greatly limits the available plate voltage swing compared to choke loaded parafeed or a conventional output transformer.

In this case I think it is a ploy mostly to save the cost of a real output transformer designed to operate at these voltages. I am currently working on a fairly conventional GM70 amp design and have some trouble believing the claimed output power ratings for this topology - the lamp loaded ones I have encountered produced something like 1/4th the power of a conventional choke loaded parafeed or conventional output transformer. (RC loading will give you no more than 1/2 the voltage swing available with an inductive load hence the much lower power output)

Mine incidentally is good for not much more than 20W with 1kV @ 120mA (calculated) and has a conventional 7K:8ohm OPT.

[DF96]

Could this start a new hobby of lamp rolling?

Whoever wrote the blurb forgot to mention that any flickering of the lamp in normal operation is a sign of LF distortion and smearing. Normally people try very hard to eliminate storage effects from audio; here storage has been deliberately introduced. I just checked my calendar, but it isn't April 1st!

[kevinkr]

Originally Posted by DF96

Could this start a new hobby of lamp rolling?

Whoever wrote the blurb forgot to mention that any flickering of the lamp in normal operation is a sign of LF distortion and smearing. Normally people try very hard to eliminate storage effects from audio; here storage has been deliberately introduced. I just checked my calendar, but it isn't April 1st!

There was a fad here quite some years ago with parafeed tube amps with light bulb loads, they do look cool and if LF content is restricted do sound reasonably good. (The resistance of the light bulbs is highly non-linear at low frequencies though) Even Nelson Pass has done a FET amp with light bulbs..

[greenvalve]

Why not just replace the bulbs with a CCS? Tho, I am not familiar with the ones that could handle 1KV+. And looks like an easy mod to 'Power Drive', if a negative voltage supply could be found. Do you have the power supply schematic?

[FullRangeMan]

Originally Posted by greenvalve

Why not just replace the bulbs with a CCS? Tho, I am not familiar with the ones that could handle 1KV+. And looks like an easy mod to 'Power Drive', if a negative voltage supply could be found. Do you have the power supply schematic?

Hi,
I have nothing more. This is all I could get on the manufacturer site.

What is CCS ??

[kevinkr]

Originally Posted by greenvalve

Why not just replace the bulbs with a CCS? Tho, I am not familiar with the ones that could handle 1KV+. And looks like an easy mod to 'Power Drive', if a negative voltage supply could be found. Do you have the power supply schematic?

There's no point to using power drive with this design or a CCS loaded output stage as the plate voltage swing is very limited compared to a transformer or choke that will let you swing almost close to ground to close to 2X the supply rail. A CCS will address the linearity issue but would have to be able to handle nearly the full 1kV across it at maximum output level. This in no way addresses the inefficiency of the design which because it has roughly half the available voltage swing capability of an inductor loaded design (choke or transformer) is going to produce about 1/4 of the output power a conventional or parafeed design.

CCS = constant current source

[FullRangeMan]

Originally Posted by greenvalve

Why not just replace the bulbs with a CCS? Tho, I am not familiar with the ones that could handle 1KV+. And looks like an easy mod to 'Power Drive', if a negative voltage supply could be found. Do you have the power supply schematic?

Just remember on this two Polish site they are mading GM70 amps with 300B as driver with good results seems, the amps are big in size:
GM70-211 project

Wzmacniacz na GM70 - Lampa - Forum Audio - Audiostereo.pl