Cossor Melody-Maker 501AC: Variants?

[russell_w_b]

I have the version with the smaller B8A glass-pipped valves in, line-up as follows: 62TH, 6F16, 62DDT, 67PT and rectifier 66KU (which I believe are 'badge-engineered' Mullard valves). However, I only have the circuit for the model with the Cossor 7S7, 7B7, etc...

Are there any differences between the other components of the Cossor Melody-Maker 501 variants? - Or would I be better-off with a cct diagram of a Melody-Maker 520, which has an identical valve line-up to my 501AC?

[quantum]

The only circuit difference between your 501 and the 520 will be with the 'tone corrector' cap in the output stage - on the 500/501 models this was a 0.005uf cap across the output transformer, and on the 520 it is a 0.01uf cap from the anode of the output valve to earth. Even that isn't critical. The main difference between the various valve line-ups concerns the value of the AF driver anode resistor, some screen grid resistor values and perhaps the cathode resistor of the output stage. Still, these sets are pretty tolerant of values - I once had a 501 which was fitted with octals but had the resistor values for the loctal range (fitted in error at the factory). The set worked and I was only apprised of the error because it seemed to have a low power handling capability before distortion set in - caused by the high value resistor fitted for the 7C6 value being now for an OM4, which with this valve resulted in a very low HT voltage. However, this set had worked throughout it's life and no-one picked up that it wrong components fitted!

[russell_w_b]

Originally Posted by quantum

The only circuit difference between your 501 and the 520 will be with the 'tone corrector' cap in the output stage - on the 500/501 models this was a 0.005uf cap across the output transformer, and on the 520 it is a 0.01uf cap from the anode of the output valve to earth. Even that isn't critical. The main difference between the various valve line-ups concerns the value of the AF driver anode resistor, some screen grid resistor values and perhaps the cathode resistor of the output stage. Still, these sets are pretty tolerant of values - I once had a 501 which was fitted with octals but had the resistor values for the loctal range (fitted in error at the factory). The set worked and I was only apprised of the error because it seemed to have a low power handling capability before distortion set in - caused by the high value resistor fitted for the 7C6 value being now for an OM4, which with this valve resulted in a very low HT voltage. However, this set had worked throughout it's life and no-one picked up that it wrong components fitted!

Thanks.

I noticed the valve line-up of my set to be identical to a 520 on a Cossor website, and started to smell a rat... Fortunately, I found a cct diagram for a 520 in Pat Hawker's red 'Radio and TV Service Manuals' to compare with the 501. I vaguely remember the chap who gave it me saying he purchased the set in 1955 as well, so I suppose Cossors took the opportunity to 'maintain the right to improve the product without further notice'

The good news is that I've got the set working fine on all bands, following a wax-capacitor replacement session and replacing the 2K7 dropper in the HT line post O/P stage. The old one was a 'body-spot-tip' type which had overheated badly.

Had it on the workshop bench for the afternoon listening to BBC Radio Cumbria on MW without problems, followed by a session at home (it's bakelite and quite hansome, so Mrs RWB doesn't mind having it on the sideboard) on BBC R4 LW. Cracking tone!

[radiogammon]

To add to the confusion, I have one which has the squared corner cabinet fitted to 520's but with a chassis and loctal valves from a 500AC and a back marked 500AC. I assumed someone had swapped cabinets at some stage, but is it possible that this could have come from the factory like this?