Old Selmer PA amp - some help reqd.

[old tube]

Happy new year to all.

I've got a 1971 Selmer 100W Pa amp that I'm trying make useable for Bass. I've read that these make great recording amps for bass.

Put a schematic below, Sorry it's not great quality but its the best I can find.

It's a 6 channel amp with 2 normal, 2 reverb and 2 "echo" channels (early effects loop) The reverb is not very good as its a small 2 spring tank, but the effects loop works very well indeed.

I picked up the amp for free as it had been lying around a luthier's workshop for about 6 years ... always going to be fixed .... It was full of dust and generally in poor shape. It barely worked, no volume and a LOT of hum. I replaced all the smoothing caps (8 of them!) plus any resistors more than 20% off spec and the bias caps. Re-valved it and "Hey presto" it worked.

It's a very, very loud amp. With guitar it has huge clean headroom and breaks up very nicely indeed when you get there, but you really need ear muffs. Very Hiwatt in its character

Here's the problem .... plug in a bass (and a 600 watt 2x10 + 1x15 bass cab) and above about half volume it distorts and not in a pleasant way .... it's not ghost notes or anything like that, it's just like the front end of the amp is being pushed too hard. It's the same in all 6 channels. I've tried reducing the gain by changing V1 with all of the 12A*7 types. Problem is that though the gain reduces ... and with a 12AU7 in V1 AND V2, it's getting there .... the volume drop is too much.

I re-biased it and replaced the output tubes with Philips 7581A's and a 12AT in the PI. While that makes it even cleaner and more headroom with a guitar, with a bass, it made no difference at all.

Is there anything I could do with the pre-amp circuit, or anything else, to "tune" it better for bass

Thanks

[roberto]

What bass and speakers cab do you use?

[Reeltarded]

You just cleaned it up on the one hand but threw out RMS potential on the other. 100w tube bass amps distort before you get to volume. They do. You can throw a signal divider before the first stage and go back with whatever the original tube compliment is for volume.

What bass? Try very low output pickups, or a clean boost as master volume before the amp. You have to make the driving signal match and that is the easy way. When you know how much clean headroom to expect after cutting the input signal you will know the potential volume you can reach before breakup.

[old tube]

Thanks for your comments.

I'm using an old Gibson Ripper bass which has fairly tame pickups at about 7.5k but they do have twin ceramic magnets. They are pretty powerful but certainly not Mud buckers. I should have said that the problems are primarily with the lower notes E and A strings, the D and G do not distort nearly as much. However the bass is professionally set up and I don't believe that a set-up issue is to blame.

The speaker cab is an old WEM Starfire 2 x 15 which I put a new baffle in to carry two Celestion 10" and one Celestion 15" speaker. They are modern high wattage bass speakers, the 10's are 150W and the 15 300W I can combine them to give 4 or 16 ohms. The speakers are tight and there is no rattle or buzz from the cab contributing to the distortion.

Reeltarded --- can you describe what you mean by a signal divider?


Thanks

[martin manning]

I think Reeltard means put a fixed volume control (voltage divider) somewhere. The question is where. You need to find out where the distortion originates, and if it's because some stage is being overdriven then you might be able to cure it that way. I'm wondering if the OT just can't handle the low frequencies, which seems to be where the issue is. It could be saturating. One thing you might try is increasing the negative feedback. That might help to keep it under control.

[Reeltarded]

Bass strings are a ton of swinging metal. 7.5k is like a medium PAF. Throw a telephone line sized wire over it and you can see much higher output than a guitar will ever make.. multiplatudinally higher. haha I love new words.

Throw a distortion unit or something in front of it with the distortion at zero, the tone in the middle and turn down the output til it cleans up. That is the amount of signal you have to lose on the front end.

Ya know..

[old tube]

Bass strings are a ton of swinging metal. 7.5k is like a medium PAF. Throw a telephone line sized wire over it and you can see much higher output than a guitar will ever make.. multiplatudinally higher. haha I love new words.

Throw a distortion unit or something in front of it with the distortion at zero, the tone in the middle and turn down the output til it cleans up. That is the amount of signal you have to lose on the front end.

Ya know..

I gotcha, too late to try it here now but i'll give it a go tomorrow. Though I suspect it won't give me the volume I seek, but I really appreciate your taking the time to help, Thanks

[old tube]

I think Reeltard means put a fixed volume control (voltage divider) somewhere. The question is where. You need to find out where the distortion originates, and if it's because some stage is being overdriven then you might be able to cure it that way. I'm wondering if the OT just can't handle the low frequencies, which seems to be where the issue is. It could be saturating. One thing you might try is increasing the negative feedback. That might help to keep it under control.

I'm a beginner at the design / alteration stuff so I'm afraid I need hand holding. I'm not fussed about altering the amp and perhaps what you're saying about the OT is true. As I say, the amp sound great with guitar, very Hiwatt. The OT is original to the amp, a Dagnall of very similar size to 100 watt Marshall's of the era but it has a 120v line out in addition to 4, 8 and 16 ohm taps.

Do bass amps as a rule have different OT's? or are things like neg feedback adjusted to optimise for low notes?

I'll try Reeltarded's suggestion tomorrow but assuming that limits the volume I can get, how would I go about altering the negative feedback?

Thanks

[martin manning]

Guitar amps don't have to reproduce signals at 40-50Hz like Bass amps do. That generally requires more iron in the OT. The easiest way I can see to try increasing NFB is to reduce the 15k resistor coming from the junction of the 2k2 and 100R at the cathode of the EF86. Parallel another resistor across the 15k, but not less than 1k 1/2 W so you don't fry the 100R. This would be for a test. A more permanent arrangement, it it looks like this is helpful, would be to re-proportion the 2k2 and 100R to keep about the same total resistance from the cathode to ground, and keep the 15k.

[old tube]

Many thanks Martin, I'll try it tomorrow and let you know.

Final question ... should I start at 1K or go a bit higher? I've got a wide selection of .5w and 1w resistors available.

[martin manning]

I guess I'd try it in steps. Maybe start with a 15k (basically doubling the FB) and work your way down...

[old tube]

Ha! went to alter the 15k resistance and fortunately measured it first ... reading 100 ohms! Shot resistor I think. Would this low a value not improve the distortion? or have I got that backwards? or could a duff resistor in that spot be causing some of the problem anyway? Thanks.

[martin manning]

Are you measuring it in-circuit? I would expect to see about 100 ohms in that case as you've got the 15k paralleled with the 100 ohm and 600 ohm through the OT secondary winding.

[old tube]

Ah! yes, .. beginners error! Sorry, rest of resistors around there all meter fine. I'll get on with adding parallel R's. Many thanks

[old tube]

Quick update.

Tried using a clean boost and while it works, its really no better as to get the volume I want, the distortion is still there.

So onto the NF

Tried 15k in parallel and it made no appreciable difference. Dropped to a 6.8k in parallel and there was definitely some taming of the distortion without any appreciable volume loss (I've put a 12Ax7 back in). Time's tight today, so for speed I went straight down to 1k. Very nice clean, tight sound, but significant volume Drop. Will try a couple of higher values tonight, looks like this may be the answer.

I'm using the 4ohm tap just now, will the effect be amplified / reduced if I reconfigured the speakers and were to use 16ohm?


Thanks