EL34, 6500A or KT88

[The Black Adder]

Hello all.

I've recently accuired a Radford STA-25 Renaissance amplifier. It's currently running EL34 of to which I've found that the literature suggests it runs otherwise with 6550A's as the output valves.

I'm new to the valve scene to the degree of power amps so can anyone give me your recommendations? I've been reading that the EL34 is indeed simular to the 6550A as well as the KT88 and that each of the latter are generally more heavy on the bass than the EL34 to which is more leaning towards the mids, can anyone confirm this for me?

Regards
Joe

[Techno Commander]

I can confirm the KT88 is a more "heavy weight" sounding valve. Whether it is a direct replacement for an EL34, I will leave to someone with more knowledge than me.

[spendorman]

As an economic replacement for my STA25 III, I used JJ KT77's can't tell the difference between them and genuine GEC KT77's (ridiculous price if one can get them now). I have the original Mullard EL34's stored away, they are still fine.

[DSJR]

The Radford under-ran the EL34's as I remember, unlike a Tube Technology Genesis mono, which doubled the output of each to make 100W and shagged them in a year. Would you need to change the spec of the EL34's, as trying to increase the ouput, assuming it was possible, would knacker the output transformers I suspect...

AnthonyTD where are you?

[The Black Adder]

Right thanks Dave n all...

I've just called Watford valve. He said that it's odd to find my model amp running EL34's and told me that it needs to be seen by someone to see if it can still run some 6550's as originally configured.

It does have a bias adjustment but he said that it's more than likely someone in the past has had a resistor changed for it to run EL34's because he doesn't think that the adjustment pots would be able to swing as much as a 12v adjustment that the 6550's need. (I think I got that voltage right)

Does this sound plausable to you guys too?

Cheese
Joe

[anthonyTD]

hi joe,
the sonic diffrences are a bit more indepth than just "one has more bass etc"
it will however be quite dependant on how the valve is run, ie; at what HT voltage, and the current drawn through the valve etc. the three valves you mention can all be run in the same circuit with the correct bias adjustment, although the output transformers will have been selected to best suit the valves that were originaly fitted. EL34 is a penode and does tend to have a leaner more forward sound in genral to both the 6550 and KT88 which are beam tetrodes, although all three can be adjusted to sound similar depending on how hard they are run, especialy in class A circuits.
the EL34 needs less bias voltage for a given current draw than both the 6550 and KT88. so if the amp in question is grid biased it would need a good variation of negative grid voltage to accommodate all three types of valves.
i hope this helps.
regards,anthony,TD...

[The Black Adder]

Hello Anthony, many thanks.

Could I give you a quick call, would you mind?

PM me your number.
Regards
Joe

[Jonboy]

HI Joe, i will hopefully be seeing a "legendary valve amp restorer" later this week and will ask him what can and needs to be done to accept the different valves you suggest if that helps.

[anthonyTD]

Hello Anthony, many thanks.

Could I give you a quick call, would you mind?

PM me your number.
Regards
Joe

hi joe,
PM sent.
Anthony,TD...

[The Black Adder]

HI Joe, i will hopefully be seeing a "legendary valve amp restorer" later this week and will ask him what can and needs to be done to accept the different valves you suggest if that helps.

Hey Jon, many thanks matey... that would be most appreciated.

Joe