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EL84 tube amp build, how to start?
EL84 tube amp build, how to start?
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Old 24th November 2020, 03:59 PM   #11
Francois G is offline Francois G  United States
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Quote:
Originally Posted by Jones880 View Post
I did find the schematic for one of the Amp's i salvaged (attached)
Good find! If your output transformers were salvaged from the radio in the schematic you can build a nice EL84 PP amplifier, even following that schematic for the amplifier part. (Did the amplifier work before you dissembled it?) Some good amplifier schematics were already mentioned, but also check diyAudio for the “El Cheapo” amp. Additionally, Dave Gillispie has nice PCBs that are upgrades to the Dynaco SCA35 that I have built and really enjoy.

Daves Store: Dynaco SCA-35 ST-35 EFB Upgrades and Restoration

The output transformers are most likely around 8k primary impedance, and obviously do NOT have UL taps. Check the transformers for continuity, at least, before investing effort. You could also verify the transformer ratio if you are up to following instructions on the web for procedures to check transformer ratio.

The 2x230 Vac power transformer is perfect for a EL84 PP amplifier. That will give you close to maximum power with fixed bias, but can also be used with automatic bias, like in your schematic, for around 10 watt output.
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Old 24th November 2020, 05:16 PM   #12
Kay Pirinha is offline Kay Pirinha  Germany
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No, vice versa. As I said before, the EL84 may perform like the EL41 under certain circumstances. Maybe that the ECL86 shows less microphony, as it is a very much newer development (one of the most recent consumer tubes in Europe) than the old EL41 from the beginning of the 1950ies.
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Old 24th November 2020, 06:04 PM   #13
andyjevans is offline andyjevans  United Kingdom
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Interesting to see the EL41 mentioned. It's a rimlock base so a bit out of the mainstream. It's a descendant of the EL33 and EL11 but not as good sounding as either of those. Sounds quite a bit like the KT61 (used in the Leak TL10 amp), but with a slight edge to it. I tried out all these tubes as drivers. An EL33 PP amp would be nice, and something I intend to build one day. But all this is going rather OT.
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Old 24th November 2020, 06:05 PM   #14
davorin is offline davorin  Croatia
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The EL80 or 6M5 is actually a NOVAL variant of the EL41, and was introduced in 1949 by PHILIPS.
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Old 24th November 2020, 07:13 PM   #15
Jones880 is offline Jones880  Belgium
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Quote:
Originally Posted by Francois G View Post
Good find! If your output transformers were salvaged from the radio in the schematic you can build a nice EL84 PP amplifier, even following that schematic for the amplifier part. (Did the amplifier work before you dissembled it?) Some good amplifier schematics were already mentioned, but also check diyAudio for the “El Cheapo” amp. Additionally, Dave Gillispie has nice PCBs that are upgrades to the Dynaco SCA35 that I have built and really enjoy.
I never tried the amp before disassembling so that is unknown. I will need to check the tube's and transformers. that they are not shorted or have discontinuities.

Rebuilding the amplifier part of this amp seems feasible, but I'm not entirely sure on where i need to cut the signal chain open to allow for a normal line-in voltage level without adding to much complexity for a first build.) wouldn't it be better to start with an amp that uses a 12ax7 as preamp and then two EL84 in PP?

thanks
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Old 24th November 2020, 07:23 PM   #16
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by andyjevans View Post
Interesting to see the EL41 mentioned. It's a rimlock base so a bit out of the mainstream. It's a descendant of the EL33 and EL11 but not as good sounding as either of those. Sounds quite a bit like the KT61 (used in the Leak TL10 amp), but with a slight edge to it. I tried out all these tubes as drivers. An EL33 PP amp would be nice, and something I intend to build one day. But all this is going rather OT.
At the start there was the EL3 and later on EL3N with new cathode technologie (Miniwatt).The EL41 is the rimlock version of the EL3N.
Mona
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Old 24th November 2020, 07:34 PM   #17
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Jones880 View Post
I did find the schematic for one of the Amp's i salvaged (attached)

it seems like the power transformer has 2x230V output with a center tab and there are multiple 6.3v outputs for the heaters of the tubes. I have no idea about the impedance of the OTs but they have a center tab at the primary side and different tabs for different impedance (4Ohm, 8Ohm, 16Ohm and 32Ohm) speakers
Looks very nice, PP transformers for the EL84.
The power transformer is a little problematic.The rectified voltage is to high,in the original circuit they loose 24V by using that for the heater of two pre-amp tubes.If you replace the diodes by a tube rectifier all is well again.But most tubes has 5V heater not present on the transformer.Can be fixed with a serial resistor or two single diodes with 6,3V heater.
Mona
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Old 24th November 2020, 08:59 PM   #18
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by Ketje View Post
At the start there was the EL3 and later on EL3N with new cathode technologie (Miniwatt).The EL41 is the rimlock version of the EL3N.
Mona
At the start of all those indirectly heated 9 watt power pentodes with comparable gm was the AL4. Even prior to this one were the directly heated RES964 and AL1 and the indirectly heated AL2. All these three had much lower transconductance, though.
Best regatds!
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Old 24th November 2020, 10:26 PM   #19
Jones880 is offline Jones880  Belgium
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Quote:
Originally Posted by Ketje View Post
Looks very nice, PP transformers for the EL84.
The power transformer is a little problematic.The rectified voltage is to high,in the original circuit they loose 24V by using that for the heater of two pre-amp tubes.If you replace the diodes by a tube rectifier all is well again.But most tubes has 5V heater not present on the transformer.Can be fixed with a serial resistor or two single diodes with 6,3V heater.
Mona
I measured the power transformer (voltages were also labeled on the trafo)
all voltages were measured unloaded
2x 130V (measured was 140v) (I misread the voltage on the schematic)
6.3v 2.2A (measured 6.69v)
6v 2.1A (measured 6.3v)
6.3v 3.1A (measured 6.71v)


I also measured the OT winding ratio and impedance by applying 6.22v (loaded) to the primary side:

the ratio to 8ohm is about 29 and the impedance is about 6.7k

so what can I do with those components?
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Old 24th November 2020, 10:44 PM   #20
andyjevans is offline andyjevans  United Kingdom
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A Push Pull output transformer with 6.6K primary is pretty standard for EL34, so I'd suggest that's the way to go. The EL34 is also good in triode. Speaking of all the older tube types, you could use an EL33 as a driver tube, sounds very nice. Something I'd like to try out since I have some. But I guess you'd probably want to use a more standard tube, like 6SN7. Or I guess you could try some 9 pin ones since you have some.

Last edited by andyjevans; 24th November 2020 at 10:50 PM.
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