HMV 801 radiogram o/p valves. - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 22nd Apr 2006, 12:40 pm

Last year I acquired an HMV model 801 radiogram. The cabinet does require some attention but the main problem with the set is that it needs two PX25 output triodes.

Genuine Marconi-Orsam PX25s do turn up from time to time but the asking price is often prohibitive. Imported examples can cost over £200 each!

So I need a substitute valve. I'm considering the EL34 connected as a triode as this valve can operate at the 425 volts HT which is the voltage supplied to the anodes of the output triodes.
The substitute valves can be mounted on a sub-assembly.
Another valve that comes to mind is the 807, but this uses a UX5 socket which can be difficult to find.

This 1937 radiogram is an impressive looking thing. The gram autochanger is the EMI K4. The audio output must be in the order of 15 watts.

DFWB

jjl · 22nd Apr 2006, 6:49 pm

The KT66 when triode connected has very similar characteristics to the PX25 according to D.T.N. Williamson's 1947 article describing the design of the "Williamson" amplifier. As EL34 and KT66 have similar ratings, I'd imagine that EL34 could be OK.
You've probably thought of this already but if you use EL34, KT66 or indeed 807 you'll need to find a 6.3V heater supply.

John

mickjjo · 22nd Apr 2006, 7:14 pm

In the magazine "Radio Bygones", issue 45, Feb/Mar 1997, P29, There is an article on the restoration of a 1940's Decca Decola, which also uses a pair of PX25s. The solution to the replacement problem in this case was a pair of 6L6s , triode connected, mounted on a plug in assembly.
For the heater supply, the 2 separate 4 volt supplies for the PX25s are connected in series and a 1.1 ohm 5 watt resistor is used to drop the voltage to give 6.3v to the parallel connected 6L6 heaters.
The new cathode biasing components are mounted on the sub-assembly and since no alterations are made to the original circuitry, The unit can be unplugged and the PX25s reinstated, Very neat, and 6L6s are still cheap.

.

Regards, Mick.

FERNSEH · 22nd Apr 2006, 10:32 pm

Hi John and Mick,
A pair of KT66s would be a good idea, after all it is a M-O valve, the same make as the rest of the valves in the set.
The other valve suggested by Mick, the 6L6 is a good idea, a low cost solution.

The two 4 volt filament supplies will be connected in series and a series resistor will take care of the excess 1.7 volts.

One thing to consider, are the KT66/6L6 valves OK at 425 volts?

DFWB

ianj · 26th Apr 2006, 6:31 pm

Ive helped restore one of these, and was very impressed with it--pp3/250s work in it, dont they?HMVs top-flight set for 1937....................ianj

Paul_RK · 27th Apr 2006, 7:59 am

Quote:

Originally Posted by ianj

pp3/250s work in it, dont they?

The PP5/400 is equivalent to the PX25: I doubt the PP3/250 would survive that kind of anode voltage for long.

I do have a lone PP5/400 here, but its status is unknown. Nice to think it might some day find a role again, if, that is, it's in usable condition. Having said that, it's a pleasant sight just standing on the shelf

Paul

FERNSEH · 27th Apr 2006, 11:24 am

It certainly would be the easy way out if a pair of PX25s, PP3/400s or the like could be found at a reasonable price.
It's going to have to be a pair of triode connected power tetrodes or pentodes, at least the long term availability of such valves can be assured.

DFWB.

Sean Williams · 27th Apr 2006, 6:54 pm

Have you asked some of the valve dealers if they have some low emmission used ones?

They dont seem to sell too well for Audiophools, and will command a lower price, whilst still working well in a radio/gram

failing that I do have some new UX5 bases if you wanted to fit triode connected 807s

Cheers
Sean

22nd Apr 2006, 12:40 pm	#1
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	HMV 801 radiogram o/p valves. Last year I acquired an HMV model 801 radiogram. The cabinet does require some attention but the main problem with the set is that it needs two PX25 output triodes. Genuine Marconi-Orsam PX25s do turn up from time to time but the asking price is often prohibitive. Imported examples can cost over £200 each! So I need a substitute valve. I'm considering the EL34 connected as a triode as this valve can operate at the 425 volts HT which is the voltage supplied to the anodes of the output triodes. The substitute valves can be mounted on a sub-assembly. Another valve that comes to mind is the 807, but this uses a UX5 socket which can be difficult to find. This 1937 radiogram is an impressive looking thing. The gram autochanger is the EMI K4. The audio output must be in the order of 15 watts. DFWB

22nd Apr 2006, 6:49 pm	#2
jjl Octode Join Date: Jan 2003 Location: Ware, Herts. UK. Posts: 1,082	Re: HMV 801 radiogram. The KT66 when triode connected has very similar characteristics to the PX25 according to D.T.N. Williamson's 1947 article describing the design of the "Williamson" amplifier. As EL34 and KT66 have similar ratings, I'd imagine that EL34 could be OK. You've probably thought of this already but if you use EL34, KT66 or indeed 807 you'll need to find a 6.3V heater supply. John

22nd Apr 2006, 7:14 pm	#3
mickjjo Rest in Peace Join Date: Jul 2004 Location: Dartford, Kent, UK. Posts: 1,661	Re: HMV 801 radiogram. In the magazine "Radio Bygones", issue 45, Feb/Mar 1997, P29, There is an article on the restoration of a 1940's Decca Decola, which also uses a pair of PX25s. The solution to the replacement problem in this case was a pair of 6L6s , triode connected, mounted on a plug in assembly. For the heater supply, the 2 separate 4 volt supplies for the PX25s are connected in series and a 1.1 ohm 5 watt resistor is used to drop the voltage to give 6.3v to the parallel connected 6L6 heaters. The new cathode biasing components are mounted on the sub-assembly and since no alterations are made to the original circuitry, The unit can be unplugged and the PX25s reinstated, Very neat, and 6L6s are still cheap. . Regards, Mick.

22nd Apr 2006, 10:32 pm	#4
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: HMV 801 radiogram. Hi John and Mick, A pair of KT66s would be a good idea, after all it is a M-O valve, the same make as the rest of the valves in the set. The other valve suggested by Mick, the 6L6 is a good idea, a low cost solution. The two 4 volt filament supplies will be connected in series and a series resistor will take care of the excess 1.7 volts. One thing to consider, are the KT66/6L6 valves OK at 425 volts? DFWB

26th Apr 2006, 6:31 pm	#5
ianj Heptode Join Date: Mar 2003 Location: Cheltenham Spa, Gloucestershire, UK. Posts: 525	Re: HMV 801 radiogram. Ive helped restore one of these, and was very impressed with it--pp3/250s work in it, dont they?HMVs top-flight set for 1937....................ianj

27th Apr 2006, 11:24 am	#7
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: HMV 801 radiogram. It certainly would be the easy way out if a pair of PX25s, PP3/400s or the like could be found at a reasonable price. It's going to have to be a pair of triode connected power tetrodes or pentodes, at least the long term availability of such valves can be assured. DFWB.

27th Apr 2006, 6:54 pm	#8
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: HMV 801 radiogram. Have you asked some of the valve dealers if they have some low emmission used ones? They dont seem to sell too well for Audiophools, and will command a lower price, whilst still working well in a radio/gram failing that I do have some new UX5 bases if you wanted to fit triode connected 807s Cheers Sean __________________ Engineers make things work and have spare bits when finished