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5.6k Output transformer for 8k 6V6?
5.6k Output transformer for 8k 6V6?
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Old 11th September 2019, 01:12 PM   #1
6V6dude is offline 6V6dude  Australia
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Default 5.6k Output transformer for 8k 6V6?

I've got a nice pair of PP output transformers for 6BM8 which is 5.6k but don't like the sound of 6BM8 tubes. Is there something I can do to be able to use them for 6V6 tubes which are 8k?
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Old 11th September 2019, 01:28 PM   #2
kodabmx is offline kodabmx  Canada
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5.6k Output transformer for 8k 6V6?
As far as I know. you can use them for 6V6 at the cost of higher distortion. You could also parallel a pair of 6V6 which would be better IMHO.
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Old 11th September 2019, 01:41 PM   #3
Kay Pirinha is offline Kay Pirinha  Germany
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A pair of 6BM8's surely will sound better @5.6k than a pair of 6V6's.
Best regards!
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Old 11th September 2019, 02:46 PM   #4
Lingwendil is offline Lingwendil  United States
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What are the ratings on the transformers?
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Old 11th September 2019, 03:14 PM   #5
Eli Duttman is offline Eli Duttman  United States
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The previously stated warnings are valid. However, something decent may be possible.

PP 6BM8s are good for approx. 8 WPC. Triode wired 6V6s will yield about the same power. Run the trioded 6V6s at lowish B+ and relatively high plate current. That will reduce the internal resistance and fit better with the O/P "iron".
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Old 11th September 2019, 06:36 PM   #6
smoking-amp is offline smoking-amp  United States
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6LU8 could be subbed in for 6BM8 with minor mods, or for just outputs only:

6AV5, 6GF5, 6EZ5, 6HE5/6JB5/6JC5, 6FW5, 12/6GE5, 12/6JN6

Check heater power available. 6V6 tubes are only rated for 40 mA DC plate current, versus 50 mA DC for the 6BM8. The subs above range from 75 mA up to 175 mA and can easily handle the 5.6K OT. 6EZ5, 6HE5/6JB5/6JC5 are closest to a supersized 6V6. 6EZ5 is pin compatible with 6V6.

6LR8 is another one, same as 6LU8 with triode driver and big pentode. 6KY8 is another possibility, with triode driver and big pentode. 6KY8 is on the $1 list at ESRC.

6MF8 has a 12AT7 like triode driver section with a 6EZ5 pentode section. On the $1 list at ESRC,
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Old 11th September 2019, 08:04 PM   #7
6A3sUMMER is offline 6A3sUMMER  United States
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First, try the amp with the 6BM8s, but connect your 6 or 8 Ohm speakers on the 4 Ohm tap (I hope you have a 4 Ohm tap). Does it sound better?

Now try the following, using 6V6 tube and the 5.6k transformer, it may work very good, but it may have one tradeoff:

Rewire for 6V6 tubes, including re-biasing (hopefully individual self bias resistors with individual bypass caps). Then reconnect your loudspeakers: If you have a 6 Ohm speaker, put it on the 4 Ohm tap. 5.6k x 6/4 = 8.4k primary If you have an 8 Ohm speaker, put it on the 4 Ohm tap. 5.6k x 8/4 = 11.2k primary

reminder: A typical 6 Ohm loudspeaker may have a minimum impedance of 5 or 4 Ohms. And an 8 Ohm loudspeaker may have a minimum of impedance of 4 or 3 Ohms. That applies, no matter what output transformer you use.

Now for the possible tradeoff: If the inductance of the 5.6k primary is too low, there will be a fall off of the lowest bass frequencies. Suppose when using the 5.6k transformer, that its inductance gives you a bass response of -3dB at 20Hz, using an 8 Ohm load on the 8 Ohm tap (-1dB @ 40Hz). With an 8 Ohm load on the 4 Ohm tap, it will be - 3dB at 40 Hz (-1 dB @ 80Hz). The 6 Ohm load on the 4 Ohm tap will be -3dB at 30 Hz (-1dB @ 60Hz). You may not miss much bass, especially if your speakers are not flat to 50, 40, 30, or 20 Hz.

Try those 6V6 tubes, the 5.6k transformers and put the speaker on the 4 Ohm tap, you just may like the sound of that.

How about a schematic?

Last edited by 6A3sUMMER; 11th September 2019 at 08:15 PM.
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Old 12th September 2019, 05:10 AM   #8
H713 is offline H713  United States
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5.6K primary... that would probably work pretty well with some 6BQ6s if you're okay running them as a pentode and in class AB. How many watts are these transformers supposedly good to?
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Old 12th September 2019, 01:24 PM   #9
6V6dude is offline 6V6dude  Australia
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These are from originally 60's HIFI amp ( I think 8 ohm) so I'd say they would be good for 12W. I all ready have stereo 6BM8 with similar OTs, I just don't like the sound of the tube, kind of sounds like EL84 with less early distortion. I don't mind them for a guitar amp but not for HiFi. I thought there might be something I could do, maybe regarding plate voltage to match them closer to 6V6. Someone here mentioned running parallel 6V6s to these OTs. I actually had these OTs in parallel 6V6 amp I've build many years ego, they're still in the amp :-) But compared to my single ended 6V6 amps I have, it's just way too inefficient for the little more power it puts out. It sounds nice and fat but classic PP would be much cleaner at higher levels.

Last edited by 6V6dude; 12th September 2019 at 01:30 PM.
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Old 12th September 2019, 04:52 PM   #10
pblix is offline pblix  United States
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Maybe the problem isn't the tube but the rest of the circuit? I made an SE amp a long time ago with 6BM8 in pentode mode and shade feedback. very similar to the RH84 amp. Worked and sounded great.
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