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Old 17th December 2015, 09:04 AM   #1
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Default Ampeg B-15 or Fender Bassman clone rebuild?

I'm a bass player - electric and double-bass, mostly jazz. I have a Selmer Treble n Bass amp which needs rebuilding, and I was wondering about changing the circuit to a Bassman (pretty close already) or maybe an Ampeg B-15. It will go in a combo with a 15" speaker.

Right now the circuit has 12AX7s and EL34s. I rather like the idea of the 1964 Ampeg B-15 with 6SL7s and 6L6s.

What would you do? Thoughts.....?
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File Type: jpg Selmer t+b.jpg (147.1 KB, 93 views)
File Type: gif Ampeg B12:B15 1964.gif (17.5 KB, 93 views)
File Type: jpg Fender B'man 5F6A.jpg (170.6 KB, 92 views)
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Old 17th December 2015, 09:10 AM   #2
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It has to be the Fender using 6L6CG output valves. I favour 6SN7 for phase splitter and driver stage but ECC83 is fine. 5U4 works well as rectifier.
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Old 17th December 2015, 09:50 AM   #3
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Quote:
Originally Posted by JonSnell Electronic View Post
It has to be the Fender using 6L6CG output valves. I favour 6SN7 for phase splitter and driver stage but ECC83 is fine. 5U4 works well as rectifier.
Tell me more - much more! I build audio tube amps with DHTs usually, so quite experienced though not an EE. I have built with all the common tubes in an audio amp and a lot of esoteric DHTs. 6SN7s are nice - a more neutral sound than 6SL7s which are more fat and warm. I'm thinking that could be good for a double bass, where warm and fat with nice sustain is a good jazz sound.

What about the output tubes? I have EL34s at present. 6L6s are less greedy on the heaters. What to expect in terms of sound and output?

I'm also debating fixed or cathode bias for the outputs. The Ampeg B-15 was rated 30w with fixed and 25w with cathode bias.

Why the Bassman over the Ampeg circuit?
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Old 17th December 2015, 10:04 AM   #4
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The Selmer is a bit toppy, the Ampeg is nice but won't go down as low as the Fender. The Bassman has a better response at lower frequencies and tends to make nice warm sounds. I would use fixed bias only because more power is available and agree 6SN7s are a great valve the 6SL7 is a bit peaky. Excellent for double bass if there is enough iron in the transformer.
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Old 17th December 2015, 10:22 AM   #5
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I was mainly thinking about the amp circuit but since you seem to know these things, what about the cabinet? I have a 15" in an infinite baffle 44x49x30cm, 65 litres. The original Portaflex was double baffle, and the designer preferred that to the B-15 Reissue which was infinite baffle, which he said "sounds a little more hollow than the thick, double baffle sound of the original B-15".

This being the case, how can I clone the bass-reflex setup of the original design - any helpful stuff online? Cabinet designs etc?
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Old 17th December 2015, 10:46 AM   #6
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Cabinets are not my thing, sorry. I am manufacturing a pair of stereo power amps using 6SL7 pre, 6SN7 phase and drive with 807 output, (push pull). Should be good for around 30W per channel. The output transformers are +- 1dB 20Hz - 20,000Hz, Ultra Linear, so like a triode output, multi-wound with a large 3" core. Fixed bias and on a 1/8" plate Aluminium case. Volume control for each channel. I will post the start to finish, when I have completed them.
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Old 17th December 2015, 12:58 PM   #7
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What about 6550 tubes? As in Marshall 1986 Bass Mk 2? The much loved Ampeg SVT used 6550s. My OPT is designed for EL34s, however, and I believe 6550 has a higher Ra
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File Type: gif Marshall 1986 Bass Mk2.gif (15.8 KB, 88 views)

Last edited by andyjevans; 17th December 2015 at 01:04 PM.
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Old 17th December 2015, 10:10 PM   #8
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You could also split the difference by using the 9 pin sockets and the baxandall B-15 circuit to essentially end up with a G-15 w/o the reverb, which works as my all-around living room amp. Having the same speaker as a B-15 (mine's a '68 or so), it does a respectable rendition of the B-15, at least at house volumes. For strict bass duty, I think you'd want use a more appropriate cab of course, perhaps a diy double baffle as stated above. Since I play both P and guitar, it serves double duty, having some of the clean attributes of a twin while still being unmistakable ampeg.. edit: and talk about your mash ups... mine's running 5881s and converted to 5AR4 rect.

Last edited by res07njc; 17th December 2015 at 10:22 PM.
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Old 24th January 2016, 07:06 PM   #9
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I've been drawing out various circuits to try to get to know them. I'm liking the output stage of the Bassman AA-864, which has a 12AT7 phase splitter. I have a bunch of 6N1Ps, and I always liked the full sound they give. So thinking of substituting a 6N1P for the 12AT7. Should be fairly straightforward allowing for the socket change and increased heater current. Incidentally, is the feedback resistor 820 in this? It's illegible.

For the input section I'm considering the B-15, either 1964 or 1966 circuit. again with a 6N1P replacement. I've attached my musings. I was further encouraged to try the 6N1P by the success of the REDDI DI box which uses it.
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File Type: png Bassman AA864 My.png (77.2 KB, 47 views)
File Type: png B-15 H'tge Circuit.png (86.4 KB, 17 views)
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Old 25th January 2016, 09:33 AM   #10
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A question for anyone who might know - I have a Selmer Treble 'n Bass 50 amp which takes EL34s and I was wondering about converting it to 6L6GCs.

I can't find any reference to what the primary impedence of the Selmer OPT is. Anyone know? Or at least can anyone guess what it is likely to be for an EL34 in fixed bias, B+ 450v?
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