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Old 30th May 2012, 03:31 PM   #21
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Guess he liked it.
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Old 30th May 2012, 05:40 PM   #22
rongon is offline rongon  United States
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Well, since you brought this thread back from the dead...

Earlier in this thread it was posited that running 6L6GC with the ST70 OPTs would result in worse performance. I was thinking that since a transformer is an impedance step-down device, what if you put your nominally 8 ohm speaker on the OPT's 4 ohm tap? That 4.3k ohm primary should now "look" like an 8.6k ohm primary, correct? I believe this halves the effective inductance of the primary, thus reducing performance at low frequencies, and reduces the output power available.

8k is mentioned as a good spot for 6L6 triode operation at lower plate voltages (say 400V plate-to-cathode). If you wanted to run 6L6 triodes into a Dyna ST70 OPT, would it be an effective strategy to connect the 8 ohm speaker to the OPT 4 ohm secondary tap?

What are the issues with this strategy?

Thanks!
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Old 30th May 2012, 08:34 PM   #23
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Primary inductance is not affeced by the choise of secondary loading.
You will however halve the power capability of the OPT. This should not be an issue when running 6L6 in triode at 400V.

I would say go ahead.

/Olof
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Old 31st May 2012, 11:38 PM   #24
multi is offline multi  Australia
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Looked up the specs for the triode connected 807 STC;{Close to 6L6 GC} at 400v Cathode bias The recommended plate load is 3000 ohms P-P. 6K p-p should work very well with 807 or 6L6GC. 807's 6L6 take a lot more drive than EL34's as triodes.
Williason used 10 K to get better Damping.
Phil
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Old 1st June 2012, 03:25 AM   #25
cerrem is offline cerrem  United States
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From all the data sheets I have seen.... The EL34 replaced the EL37 in 1954...but the EL34 was for higher voltage operation.... Looking at the plate curves and other AC parameters....gm....ect... they appear to be the same .....
I remember reading an article years ago from Mullard that discused this issue of the EL37 being discontinued and the EL34 replacing it for transmitter usage but with the advantage of higher voltage operation...

Chris
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Old 4th July 2012, 03:47 AM   #26
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Does anybody own an amplifier that was really built from new to work with the EL37?
I do and it is a bit of a pest when it comes to getting spares.
Last time i tried an EL34 it was too tall to fit my case but the sound was OK. There are a few small differences in the spec.
The older EL37s have a bigger envelope and can take a bit more before the glass melts. That is why the shape of the glass is the way it is.
If anyone wants help with guitars i would have a look at EL34s with international mains transformers for the output stages as the guys that play these instruments are more interested in the way the amplifier goes into clipping when worked hard.
Most of these guys are using EL504s with top anode caps this side of the water and also they are being manufactured again too. They sell fancy anode cap insulators for them.
Thin laminates in the output transformers is the best way to a true HI FI amplifier anyway before you choose the tubes.
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Old 24th December 2012, 01:22 PM   #27
GordonW is offline GordonW  United States
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Quote:
Originally Posted by hemgjord View Post
Primary inductance is not affeced by the choise of secondary loading.
You will however halve the power capability of the OPT. This should not be an issue when running 6L6 in triode at 400V.

I would say go ahead.

/Olof
Just ran across this thread- there is one more factor, besides just the inductance, in determining performance of an output transformer, being used with impedance other than specified.

The LF cutoff of a transformer is a function of the inductance AND the output load. Just because a transformer has a certain inductance, it will NOT mean the transformer will have the same good LF performance, if the load is changed.

In fact, using a transformer, with an 8 ohm load on a 4 ohm tap, will usually degrade ultimate low-frequency performance. It will probably lose about a half-octave or so, of LF extension at full power.

OTOH- if the primary and secondary can handle the extra current- doing the opposite- running a 4 ohm speaker on an 8 ohm tap, or an 8 ohm speaker on a 16 ohm tap- can sometimes lead to an extension of the useful LF limit of the transformer, by up to a half octave.

Of course, there are other factors- including resonances, capacitance effects and such, which may mean it's not quite as simple as changing the load to half nominal value. You may run into situations, where the stability of the amp can be compromised, due to compensation networks and transformer resonant effects which may not be optimal with the new loading, without other changes in the amp as well. But, as seen by the Tronola article "Hotrodding the Eico ST70" (seen here: Hotrodding the Eico ST-70 ), it can sometimes work!

Regards,
Gordon.
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