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Very dumb question about EL84
Very dumb question about EL84
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Old 4th June 2019, 09:52 AM   #11
KodzghlyCZ is offline KodzghlyCZ
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Quote:
Originally Posted by SemperFi View Post
Nah for a simple amp just to start off this is fine. But coupling caps should be reduced to 10-47nF and not be 470nF. And the grid stopper on the EL84s increased to 10k-100kohm (experiment a little).

This schema is for a HiFi amp. The amp will lack in the gain department, not possible to over-drive, but I assume after a successful build, an additional gain stage can be added.

I'm mostly worried that a guy who doesn't know ohms law is building something from scratch. But if he's a handy guy he may just build fine and hopefully finds the motivation to learn what needs be.
I have separate schematics for a guitar preamp with overdrive and tone controll, so I don't need overdrive on this much.
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Old 4th June 2019, 11:42 AM   #12
Zung is offline Zung  Switzerland
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Quote:
Originally Posted by SemperFi View Post
...
I'm mostly worried that a guy who doesn't know ohms law is building something from scratch...
I was 10 when I built my 1st amp: a single ended ECL82 thinggy, point to point. I was fairly proficient at counting (to 10) at that time, but ohms' law had to wait another 10 years...

Quote:
Originally Posted by KodzghlyCZ View Post
I have separate schematics for a guitar preamp with overdrive and tone controll, so I don't need overdrive on this much.
Nah, the beauty of a tube amp is when it's fully saturated, from preamp to power amp to output transformer. No gadget I know comes even close. But then I have to add a power attenuator to protect my ears.

Note that if you use point-to-point, it's not a big deal to rewire the thing to whatever you fancy.
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Old 4th June 2019, 06:14 PM   #13
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by KodzghlyCZ View Post
so I don't need overdrive on this much.
Are you planning to use this along with an external preamp? Because the circuit you showed has no volume control, no tone control, and I don't think it has enough gain even for clean tones.

The input valve (6N1P) seems to have a u of only 33, so the voltage gain of that stage will be less than this; probably between 20 and 30 times (roughly 26 dB and 30 dB.)

The cathodyne phase splitter has unity gain (0 dB).

The output stage usually ends up having roughly unity gain - there is voltage gain from the EL84s, but then there is voltage loss in the step-down output transformer. Overall, the voltage gain tends to be not too far from unity (0 dB), give or take a few decibels.

So the entire amplifier only has a gain of about 26 - 30 dB, or 20 - 30 times. If you assume 20 mV from the guitar, that is only enough gain to output about 400 mV - 600 mV across the loudspeaker.

You can certainly hear that, but it will be quiet - only a few tens of milliwatts. Far from what the amp is capable of.

At full power, that EL84 push-pull output stage will typically be able to deliver something close to 10V RMS, or roughly 30 volts peak-to-peak. To do that from 20 mV input, even if we assume the 20 mV is RMS, you need a voltage gain of about 500x - 1000x (34 - 40 dB).

Unless you're planning to use a separate preamp, I would say your amp needs 10 - 20 dB more voltage gain, even for clean tones with a typical guitar, especially a guitar with single-coil pickups. One more 6N1P gain stage at the input would probably do the trick.


-Gnobuddy
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Old 4th June 2019, 08:33 PM   #14
KodzghlyCZ is offline KodzghlyCZ
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Join Date: Jun 2019
Quote:
Originally Posted by Gnobuddy View Post
Are you planning to use this along with an external preamp? Because the circuit you showed has no volume control, no tone control, and I don't think it has enough gain even for clean tones.

The input valve (6N1P) seems to have a u of only 33, so the voltage gain of that stage will be less than this; probably between 20 and 30 times (roughly 26 dB and 30 dB.)

The cathodyne phase splitter has unity gain (0 dB).

The output stage usually ends up having roughly unity gain - there is voltage gain from the EL84s, but then there is voltage loss in the step-down output transformer. Overall, the voltage gain tends to be not too far from unity (0 dB), give or take a few decibels.

So the entire amplifier only has a gain of about 26 - 30 dB, or 20 - 30 times. If you assume 20 mV from the guitar, that is only enough gain to output about 400 mV - 600 mV across the loudspeaker.

You can certainly hear that, but it will be quiet - only a few tens of milliwatts. Far from what the amp is capable of.

At full power, that EL84 push-pull output stage will typically be able to deliver something close to 10V RMS, or roughly 30 volts peak-to-peak. To do that from 20 mV input, even if we assume the 20 mV is RMS, you need a voltage gain of about 500x - 1000x (34 - 40 dB).

Unless you're planning to use a separate preamp, I would say your amp needs 10 - 20 dB more voltage gain, even for clean tones with a typical guitar, especially a guitar with single-coil pickups. One more 6N1P gain stage at the input would probably do the trick.


-Gnobuddy
Yeah, I have an external preamp with equalization and little bit of a drive. Recently i Found some schematics, that are complete, even with the preamp stage and they are directly from tube guitar amps, so those might work good. Thanks for replying
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Old 10th June 2019, 08:54 PM   #15
Psicopanque is offline Psicopanque  Brazil
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go for 135r/10w wirewound resistor in the cathode of both el84 if strapped

or 270r/10w for each el84 if you dont plan to share resistors on cathode, using independent biasing for each tube

both are acceptable, try it and see...

the cathode caps, go for 1000uf as in the schem for shared/strapped cathode... if each tube get its own, then 500uf or 1000uf for each, both are good;

if you like smooth cleans without harsh treble peaks, then lower the grid leaks from 470k to 220k and increase the grid stoppers from 1k to 10k... i did it on my own amp and the punch remained very close to the same as before... it helps alot if you like alot of preamp drive

i dont know about the PI before the el84's on the schem, for me its always been the long tail pair as our fellow previously said... maybe you should try it?

if you like to play guitar on superlead voicing, maybe you should unbalance it a little with assymetrical grid leaks on the el84 (220k/330k or something like it) but you must do it only if the cathode is shared between both el84's, yet remember that this tweak is only fun to do if you use very little preamp saturation with a hot pickup such as the JB or something

i hope the advice helps, since no one said it before... thats just my 2 brazilian cents, they are worth less than 1 cent in north american and european currency... worthless HaHhaHaha
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