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Old 31st December 2003, 05:36 PM   #1
rvas18 is offline rvas18  United States
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Default turning a SE 6v6 to a 6bq5 amp

I am building my first amp soon and I have been working with my schematic. I know that the 6v6 and the 6bq5 tubes are similar in many ways but I am not knowledgeable enough to modify the values on the schematic myself. I modified the original by removing the feedback, I added a self-biasing cap to the 12ax7, and I think I correctly added a switch to go between triode and pentode mode (the original was drawn for pentode mode). It is necessary that I use the 6bq5 and the 12ax7 because I already have a good number of each tube on hand. The transformers were salvaged from an old RCA SE console unit. I would appreciate any help on turning this schematic to accommodate a 6bq5 tube, and also if possible suggestions on where and what parts to get for the coupling caps, resistors, and bias caps. I am not completely set on using this schematic either, if ananyone knows of a 12AX7, 6BQ5 SE amp that will obviously outperform the one I am trying to configure please post a schematic.



Thanks,
Raśl
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Old 31st December 2003, 10:04 PM   #2
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Hi,

Hi,

The EL84/6BQ5 is a drop in replacement for its granddaddy, the 6V6.

Once you have the noval socket in place just check the pinout changes if any, (I didn't check) and it should work.

Cheers,
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Old 31st December 2003, 10:22 PM   #3
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I've usually seen higher value cathode resistors for the 6V6 than for 6BQ5 in SE. Can't see your schematic from the link, as you apparently to be a member.
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Old 1st January 2004, 02:10 AM   #4
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Quote:
Originally posted by fdegrove
Hi,

Hi,

The EL84/6BQ5 is a drop in replacement for its granddaddy, the 6V6.

Once you have the noval socket in place just check the pinout changes if any, (I didn't check) and it should work.

Cheers,

Erm... maybe if you drop the cathode bias by about half... but then, that's obviously not plug-in... It's got a lot more Gm.

Tim
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Old 1st January 2004, 10:30 AM   #5
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Its true that a 6BQ5 is a 'beefier' tube than the 6V6, and ideally it should have a resistor for cathode bias of about 150 ohms as opposed to the 240 ohms in the circuit diagram, but it will work in this circuit as a direct replacement - you could argue that the higher cathode resistor will reduce the gain/output of the 6BQ5 to nearer the levels of a 6V6. Since in optimum conditions 6BQ5s take a higher HT current you might not want it in such conditions, as it increases demand on the power supply - the extra heater current will be putting greater demand on the power supply anyway.

On a couple of occasions in old tube sets I've replaced the 7C5 (the loctal equivalent of the 6V6) with a new socket and a 6BQ5 without making any other circuit alterations (apart from changing the dial lamps for ones with a lower current rating to reduce demand on the mains tranny) and they work OK - it theory it may be bad practice but in reality there is no problem.

With the circuit given, one addition I would make would be to add a small cap across the primary of the output trannies to reduce third harmonic high frequency transients when run in pentode/tetrode mode, as there is no negative feedback to reduce this.
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Old 2nd January 2004, 08:01 PM   #6
rvas18 is offline rvas18  United States
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I have taking many of the suggentions i have gotten and reworked the schematic onve again. I have lowered the cathode resistor to 150ohms, and implemented a swithced on\off negative feedback, and decreased the input pot from 500ohms to 100ohms. Because this is my first amp i would like to be able to use try all the different configs (no NFB\NFB, triode\pentode, etc) without having to build several different amps.

i have another possibly quick question, a friend of my dad gave me some more vintage tubes, i got 2x 12h6, 4x 12SL7GTA, 6x 12AU7A, 2x 6AQ5, 6x 6X4, and 6 more 12AX7A's are any of those useful for audio purposes, and if any are can they make a higher quality amp than the 12AX7\6BQ5 amp i am configuring now?
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Old 2nd January 2004, 08:05 PM   #7
rvas18 is offline rvas18  United States
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for you non-members to view the schematic: http://www27.brinkster.com/gulgcoast/schematic.html
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Old 3rd January 2004, 11:55 AM   #8
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I don't think that the 6AQ5s would give you any advantage over the 6V6s you started with in the circuit you are using.

With the second schematic you've posted, I assume that the 150 ohms cathode resistors in the output stages should be for the 6BQ5s and not the 6V6s? Also, the 330mfd caps across the cathode resistors in the 12AX7 stages would negate the effectiveness of the negative feedback arrangements.
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Old 4th January 2004, 01:36 AM   #9
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The 6BQ5'S will draw almost twice the fillament current as the 6V6'S ,if the transformer powered other tubes in the stereo it will be ok ,but if only powered the tubes shown you might want a seperate fillament transformer .
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Old 4th January 2004, 05:58 AM   #10
rvas18 is offline rvas18  United States
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i plan to use the 6v6s now
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