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Thoughts on a tube sub...
Thoughts on a tube sub...
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Old 19th September 2019, 08:03 PM   #11
smoking-amp is offline smoking-amp  United States
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The 7054 has higher gm than the 6CL6 for the same current, about 1.3X. So I would increase the 80 Ohm cathode resistors to 100 Ohms as a 1st guess to get the same driver stage current.

The Sweep tubes will only need about -30V to turn them off versus -50V for the 6146x tubes (depending on the Vg2 used, assuming 75% to %60 lower Vg2 for the Sweeps). You may be able to reduce the driver current a little further and increase the 8200 Ohm driver loads some, so less heat developed there. Could try 120 Ohms for the driver cathode R and 10K for the driver load R, see if the driver is developing enough swing and is centered in the available driver V range.

Last edited by smoking-amp; 19th September 2019 at 08:23 PM.
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Old 19th September 2019, 08:38 PM   #12
smoking-amp is offline smoking-amp  United States
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The Sweep tube grid 1 bias resistors can be increased above 22K, more like 100K.

That allows increasing the driver load R some more, with a similar reduction in driver current.
Maybe go to 20K for the driver loads now, with 180 Ohm driver cathode R (for bias ). Oh yeah, the 330 Ohm driver tail R would need to be increased similarly to 750 Ohm or 820 Ohm. An adjustable CCS tail from -Vbias would allow setting the driver stage current better, with lower distortion too.

Last edited by smoking-amp; 19th September 2019 at 09:06 PM.
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Old 20th September 2019, 02:41 AM   #13
PRR is offline PRR  United States
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Ampeg SVT bass-amp, early production, used a sextet of 6146 and delivered AMPLE bass. It is a truly frightening beast.
https://el34world.com/charts/Schemat...odel_6146b.pdf

After a few years Ampeg switched to selected 6550, and these are arguably more reliable.
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Old 20th September 2019, 03:56 AM   #14
smoking-amp is offline smoking-amp  United States
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With the RCA 6146x datasheets listing absolute max Pdiss (20 or 27 Watt) instead of the design max, I guess there was some tendency to push the limit in designs. The 6146 plate looks similar in size to a 6JN6/6GE5 Sweep tube (17.5 Watt).

The 6146 datasheet lists 30,000 Ohms (instead of 0.5 meg Ohms for class AB1 ) for a class AB2 grid 1 bias resistor. Since the schematic has 25000 Ohm grid 1 resistors, and around 150 Watt output (OT rating), this design must have been running class AB2. One would expect a grid 1 cooler fin up top for class AB2, but the 6146 has an RF shielding box up top instead. The grids probably get really hot in the 6146 tubes, and then runaway....

For running class AB1, the driver load resistors could be increased even further beyond 20K. (with similar changes to the driver cathode R and tail R) One needs to check the input capacitance of the Sweep tube versus the doubled 6146 to see how far that can go before HF bandwidth is an issue.

With a single set of 6JN6/6GE5 I would think 50 Watts out max would be more typical in class AB1. Two sets, 100 Watts. With some big TV Sweeps (26LX6, 36MC6, 26LW6...), maybe 150 Watts in class AB1. Class B could get more. Tubelab I think has gotten up to near 250 Watts with the _LW6 at 70% plate efficiency.

Last edited by smoking-amp; 20th September 2019 at 04:25 AM.
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Old 20th September 2019, 04:50 AM   #15
smoking-amp is offline smoking-amp  United States
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That Ampeg SVT 300 W Amp must be running near class B2 with -47V bias and cathode follower grid drives. 47K in series with the 6146B grids must produce some top end distortion, but maybe limits runaway.

Last edited by smoking-amp; 20th September 2019 at 04:52 AM.
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Old 21st September 2019, 12:46 AM   #16
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For the original circuit, if you are going to be using say an Edcor 100 Watt 4K OT, there is no need to keep the B+ up at 600V, that will just heat the tubes un-necessarily.

500V B+ will drop the power output level to 100 Watts, and 450V B+ will drop the power level to 80 Watts, with cheaper capacitors for the power supply. A little safer to work on too.

Shifting to a 3.3K 100 Watt OT would want around 460V B+ for 100 Watt output. Not counting OT losses in these estimates.

You should not need to go grid positive (AB2) with big TV Sweeps for a 100 Watt power output level.

Last edited by smoking-amp; 21st September 2019 at 12:54 AM.
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Old 21st September 2019, 05:00 PM   #17
pblix is offline pblix  United States
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For a new build I suppose the 6JG5 could be a substitute for 6CL6. Another benefit is that it is on the ESRC $1 menu. Lower heater current but less dissipation. Just throwing it out there.
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Old 21st September 2019, 05:47 PM   #18
Brice is offline Brice  United States
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Cool schematic. Thanks for sharing.
Please show us pics of the build.
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Old 19th October 2019, 04:49 PM   #19
rsumperl is offline rsumperl  United States
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Thoughts on a tube sub...
Good Saturday everyone,
Any thoughts on using a 7645, 6360, or a 6939 in place of the two 6CL6s?

Thanks
Ray
Attached Files
File Type: pdf 7645.pdf (503.6 KB, 2 views)
File Type: pdf 6360.pdf (994.7 KB, 2 views)
File Type: pdf 6939.pdf (1.42 MB, 3 views)
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