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RE: Bogen HO50 1949 AB2 vintage amp

Posted by ivan_terrible on October 19, 2020 at 01:52:22:

This is how a modified one looks underneath.
It's a little difficult to explain how to move from dropping resistors on the original amp, which is fundamentally unstable to a much better solution with modern zeners.
A shunt stabiliser is run off the main 500V HT via the original dropping resistor chain (which used to be across both the screen and anode supplies to ground).

The visible wire ended one under the chassis is run immediately at the full max of 0.040A in series with another one above chassis in a loctal base, where the original 5R4GY used to sit.
This NOS one was from the BBC from years ago...
As the anode supply draws down from its initial 600V+, (as the output valves warm up), and the AF amp valve heater warms up, the gas discharge valves pull back nicely to a more moderate 0.020A, and the new HT supply settles at 505V, a far more RIGID 500V than ever before despite having 32V drawn off it on the string of zeners in the CT return.

The 5R4 was moved from its original spot to next door instead of the 5Y3, 'cos of course it provides a much lower voltage drop for a screen supply than the 5Y3, and gives a warm up gentle start.
That was also fed with silicon diodes and the ones in the 5R4 put in parallel.
This is quite a smart mod.
The main Anode supply comes straight out of some silicon diodes into the swinging choke, then straight into the dropping resistor down into the gas stabilisers.
There's no non linear voltage drop thru a valve rectifier.
They act as surge limiters, by lighting up straight away, and also giving an immediate return current via the zener diodes in the CT of the main anode supply.
That brings the 32V bias supply up immediately well before we start the screen grid supply up.
So the gas discharge valves, derive the stabilised HT supply for the AF amp section (minus hum and noise).
Originally all that was derived via a dropping resistor from the screen supply.
Clearly as the screen/driver current varied, so did the supply to the AF amp.
807s and KT8 have aligned screen grids, so in reality the screen supply demand is not enormous.
Here's some comparisons of how the original amp fared at merely 15W (distortion started to rise rapidly), compared with how it now manages a really linear 60W after being sorted.
There were some horrible crossover distortions and instabilities taking place especially between 20-100hz...and nowhere near the 50W it was supposed to make.

The paraphrase invertor was clearly not happy, needing a balance pot to make it better.
Most people would of course have given up here, and said, "NAH its just a rubbish 1940s PA amp init, something about silken purses and sow's ears?"
100hz?

50hz?

Well, if you gave up then that you would be wrong.
Bogen have excellent output transformers wound themselves on Selectron cores, all that was really needed was balancing the phase invertor correctly, stabilising the power supply, and REDUCING the original amount of NFB which was overloading one side of the AF amp while giving no help to the other side which wasn't.
Sound familiar.
It's the main reason why people threw Bogen amps in the bin claiming they could never be any use for hifi.
So, take a look at this after well sorted.

60W square wave?

70W 1khz sine wave?

Who says AB2 amps based on the 807/KT66 are dirty with gross distortion?
This Bogen is an absolute gem and it's 70yrs old!
A ruler board frequency reponse dead flat from 30hz-15khz, with no sign of instability into any load?

The irony was not lost.
The shop where we tested it, didn't like the sound.
"too neutral" they said, "too clean"..another said.
What to say?
They loved the sound of the original 6V6 powered DB110, which was way less powerful, extremely coloured with a great big lump in response at 100-300hz typical of valve amps!