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Old 6th May 2006, 04:45 AM   #1
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Default My El34 SET

I changed from the 6l6GC to the EL34 because It seemed like a better tube for me. it had more power in Triode form, and A higher plate voltage. However I wanted to ask you guys what you think of EL34s as an SET because I have noticed some things very odd about my amp

The bass response is rather excellent, it really thuds

however, the higher bass spectrum (probablly around 100hz-500hz) sounds really poor when the volume is about three quaters of the way up.

I am using cheap Hammond 125CSE transformers, and cheap out put caps, are my problems in those cheap components?

Will I be happier investing a good 40 dollars in slightly less cheap center-tapped transformers to change to UL or Pentode? Any help would be much appreciated
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Old 6th May 2006, 07:48 AM   #2
vax9000 is offline vax9000  United States
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You may want to try a smaller coupling capacitor. Your bass will be a little bit weaker, but you will be able to avoid the clipping_at_low_freq problem.
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Old 7th May 2006, 05:53 PM   #3
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Default more info

It's hard to give advice with so little information and most everything is so subjective in listening, especially when no measurements are given.

The poor frequency response as well as the good response at various frequencies may not be soley due to electronic components. Room effects can have significant impacts on frequency response, too.

The caps and OPTs can have major affects on frequency response, no doubt. IME the OPTs have the most significant effect assuming the caps are sized properly, within the context that you have provided. Better trannys than those you described will give obvious improvements.

HTH
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Old 8th May 2006, 03:28 AM   #4
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Sorry about the low information level, I'll explain some more right now.
here is a look at the schematic with the preposed solution, using an NFB Loop with a Pot to limit the negative feedback, which it what I think the problem is
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Old 8th May 2006, 05:31 AM   #5
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Quote:
Originally posted by alexmoose
Sorry about the low information level, I'll explain some more right now.
here is a look at the schematic with the preposed solution, using an NFB Loop with a Pot to limit the negative feedback, which it what I think the problem is

Try removing the bottom of the 1000 uF output tube cathode R bypass cap from the speaker output terminal and placing it to power supply ground. You could also try much smaller cap sizes here. 100-500 uF.
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Old 8th May 2006, 07:12 AM   #6
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What rcavictim says is right.

With the EL34 cathode bypass cap wired to the speaker output terminal like this you have a reasonable level of feedback around the output stage itself and the driver is starting to strain under the output tubes increased drive requirement. This will show up mainly at higher volume settings.

At say 5 Watts Output you are having to swing an extra 6 V RMS (nearly 18 V pk - pk) to overcome this negative feedback.

If you want to keep that quite elegant scheme then you'll need to beef up the driver.

Is there a 4 Ohm - ish tap you can attach the bypass cap to instead?
That would still give you some feedback at the output tube (70% of what you had) and ease the loading on the driver a bit.

Cheers,
Ian
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Old 8th May 2006, 11:54 AM   #7
ilimzn is offline ilimzn  Croatia
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Why the diode vrom B+ to the primry of the output transformer???
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Old 8th May 2006, 04:33 PM   #8
Svein_B is offline Svein_B  Norway
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Just a thought ?

Have you measured (or listened to) the output signal level with and without the two different feedback loops to make certain that both are in fact negative ?
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Old 8th May 2006, 06:38 PM   #9
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An easy way to get more drive... increase the EL34 grid resistor and/or decrease the 6AQ8 load and cathode resistors... 100K is a bit high for that tube... distortion, gain and high frequency response will improve with a lower value.
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Old 8th May 2006, 09:30 PM   #10
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Quote:
Originally posted by gingertube
What rcavictim says is right.

With the EL34 cathode bypass cap wired to the speaker output terminal like this you have a reasonable level of feedback around the output stage itself and the driver is starting to strain under the output tubes increased drive requirement. This will show up mainly at higher volume settings.

At say 5 Watts Output you are having to swing an extra 6 V RMS (nearly 18 V pk - pk) to overcome this negative feedback.

If you want to keep that quite elegant scheme then you'll need to beef up the driver.

Is there a 4 Ohm - ish tap you can attach the bypass cap to instead?
That would still give you some feedback at the output tube (70% of what you had) and ease the loading on the driver a bit.

Cheers,
Ian
There is a 4 ohm tap on the transformer, whats the worst that could happen If i try it?


Quote:
Originally posted by Svein_B
Just a thought ?

Have you measured (or listened to) the output signal level with and without the two different feedback loops to make certain that both are in fact negative ?
No, I haven't listened to both, just to the one without the feedback loop, I have not implimented it yet, however I do not know if it is infact negative, how would having positive feedback differ?

Quote:
Originally posted by Tom Bavis
An easy way to get more drive... increase the EL34 grid resistor and/or decrease the 6AQ8 load and cathode resistors... 100K is a bit high for that tube... distortion, gain and high frequency response will improve with a lower value.
What values were you thinking?
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