Underperforming valve head

[razze06]

[quote name='Bassnut62' post='1152765' date='Mar 7 2011, 06:20 PM']Bassman amps are reknowned for great tone and incredible clean all the way to 11 on the dial. I don't reckon your amp should be distorting at all unless you put a dirt box in front of it. I guess there could be lots of reasons for your probs; but I think only a tech will be able to tell if there is a valve, component or clone-design fault with the amp.

I absolutely know what you mean about Bassman tone too - it's a thing of great beauty.
So worth persuing; but maybe you would be better to sell this clone and one of your other amps and go out and buy a real Bassman and go for the 135 this time.[/quote]

Well, looks like reasonable proper bassman 135 etc start at £350 or so. This head is probably worth what, £150 or so? Can't really afford the difference right now...

[razze06]

[quote name='Bassnut62' post='1152765' date='Mar 7 2011, 06:20 PM']Bassman amps are reknowned for great tone and incredible clean all the way to 11 on the dial. I don't reckon your amp should be distorting at all unless you put a dirt box in front of it. I guess there could be lots of reasons for your probs; but I think only a tech will be able to tell if there is a valve, component or clone-design fault with the amp.[/quote]

I wonder if the guy that put this together somehow altered the design to allow more overdriving of the front end. If that's the case, I'm sure it can be reverted or altered. I never realised that bassman amps were not supposed to overdrive until the very limits of their operating range.

Time to take it to a good tech!

[dan670844]

[quote name='razze06' post='1148112' date='Mar 3 2011, 10:28 AM']Thanks for the replies.

- I have another couple of amps and cabs that I regularly use for gigs and practice, so I don't really need another amp. I like the tone to bits though, and I'd love to use it more.

- According the notes scribbled on the backplate, the amp is designed to take 4 or 8 ohm loads. I never put more or less, or even in between (mixing 4 and 8 ohms)

- I would have thought that 100W would be plenty loud. For a couple of years I gigged and practiced with a 120W hartke kickback combo, so I think i'm entitled to expecting more sound out of this setup. We're not THAT loud...

- It does have a preamp section, and if I crank both pre and power section to full I get overdrive from hell. Actually, you get Motorhead Lemmy's tone by cranking the power section to full whack and about 1/3 of the way up on the preamp gain. Don't know it the pots are linear, i suspect not...

- Maybe i'll phone up Dennis Marshall, or some other tech. Big_stu, what's you real name, so that I can say hi on your behalf?

Keep them coming, I'd love to get this monster to its full potential![/quote]



1. Check all the valves are ok i.e preamp and power
2. As others have said check the bias current / balance check if you are checking enough current to those 6l6's so they are putting enough juice out
3. Check that pots and the input jack plug are ok
4. Check the Output transformer (dont touch the HT wires Hahaha) what are the taps off of it? is it wired correctly i.e and output to 4 or 8ohm, does it have a selector switch to switch impedance. If it only has one tap (i.e only two wires coming off) off the transformer to one jack plug and no selector then it is set to only one impedence, which will be only 4 or 8 or 16ohm, so you cannot run any other cab that what it is tapped to i.e 8ohm = 80hm cab or two 16ohm (haha if you can find them). A lot of the old bassmans only had 4 0hm taps to run 2x 8ohm cabs. check that output transformer is also up to the job, most ot for bass are over rated so for a 100watt amp they are usually 150-200watt output
5. Check the mains transformer, can it supply enough current for the job?

That is really it if it sounds ok and it was built from a kit, like torres, TAD etc then the parts will be up to the job, so its more likely that the bias current is not high enough so the valves are not putting out enough current into the output transformer.

Good luck
D

[dan670844]

[quote name='dan670844' post='1155099' date='Mar 9 2011, 11:37 AM']1. Check all the valves are ok i.e preamp and power
2. As others have said check the bias current / balance check if you are checking enough current to those 6l6's so they are putting enough juice out
3. Check that pots and the input jack plug are ok
4. Check the Output transformer (dont touch the HT wires Hahaha) what are the taps off of it? is it wired correctly i.e and output to 4 or 8ohm, does it have a selector switch to switch impedance. If it only has one tap (i.e only two wires coming off) off the transformer to one jack plug and no selector then it is set to only one impedence, which will be only 4 or 8 or 16ohm, so you cannot run any other cab that what it is tapped to i.e 8ohm = 80hm cab or two 16ohm (haha if you can find them). A lot of the old bassmans only had 4 0hm taps to run 2x 8ohm cabs. check that output transformer is also up to the job, most ot for bass are over rated so for a 100watt amp they are usually 150-200watt output
5. Check the mains transformer, can it supply enough current for the job?

That is really it if it sounds ok and it was built from a kit, like torres, TAD etc then the parts will be up to the job, so its more likely that the bias current is not high enough so the valves are not putting out enough current into the output transformer.

Good luck
D[/quote]

Also check the preamp valve, if it is a total clone V1 and V2 sound be 7025 and V3 should be a 12AT7, sometimes guitarists get hold of these and use them a guitar amps, they might have messed with the preamp and driver valves

[razze06]

Got the initial response from the tech:

- the amp dates from 74-75, given the close spread of dates on the components

- the components are the best you could buy at the time

- there are a number of small mistakes, all from the original design, which may have cause some problems in the OPT. Still investigating this aspect.

- There are no later mods, so all problems seem to be inherent with the original design

[Ancient Mariner]

Interesting, thanks for keeping us up to date.

[Bassnut62]

[quote name='razze06' post='1154988' date='Mar 9 2011, 10:00 AM']Well, looks like reasonable proper bassman 135 etc start at £350 or so. This head is probably worth what, £150 or so? Can't really afford the difference right now...[/quote]

I was just thinking that you might end up having to spend over £100 to fix this amp, so you may end up being £300 into this clone before you know it; in which case you are close to cost of a real Bassman.
Either way good luck on your journey, which I reckon is a good one. Bassman heaven is a thing of beauty IMO.

[umph]

[quote name='Bassnut62' post='1152765' date='Mar 7 2011, 06:20 PM']Bassman amps are reknowned for great tone and incredible clean all the way to 11 on the dial. I don't reckon your amp should be distorting at all unless you put a dirt box in front of it. I guess there could be lots of reasons for your probs; but I think only a tech will be able to tell if there is a valve, component or clone-design fault with the amp.

I absolutely know what you mean about Bassman tone too - it's a thing of great beauty.
So worth persuing; but maybe you would be better to sell this clone and one of your other amps and go out and buy a real Bassman and go for the 135 this time.[/quote]

i've never had a bassman that stayed clean over half way over the dial

[Mr. Foxen]

[quote name='Bassnut62' post='1167922' date='Mar 19 2011, 06:29 AM']I was just thinking that you might end up having to spend over £100 to fix this amp, so you may end up being £300 into this clone before you know it; in which case you are close to cost of a real Bassman.
Either way good luck on your journey, which I reckon is a good one. Bassman heaven is a thing of beauty IMO.[/quote]

Thing is, it will be fully teched and sorted. Fender QC was dodge from the factory, and a £300 bassman probably won't be tip top.

[Bassnut62]

[quote name='Mr. Foxen' post='1168259' date='Mar 19 2011, 02:14 PM']Thing is, it will be fully teched and sorted. Fender QC was dodge from the factory, and a £300 bassman probably won't be tip top.[/quote]

You're probably right. A tip top one would probably be over £400

[razze06]

[quote name='Bassnut62' post='1170085' date='Mar 20 2011, 09:37 PM']You're probably right. A tip top one would probably be over £400[/quote]

I agree. Moreover, it can be a little hard to tell if it's in tip top conditions until you bought it and tried it in a real band situation. That can be hard unless it's local or from a fellow BCer who knows his/her amps.

The clone I have has a lovely tone, so at least I know it's right in that department. If the problem is "just" with the power stage, odds are the repairs will let me keep the tone I like, and give me enough power to gig and practice with it!

If it's a new OPT, £100 for the tranformer and another £60 in labour. I'd be happy with that.

[razze06]

Looks like this is an extra complex job! apparently, the power stage is based on the bassman 100 schematics, but the pre is more like a fender twin. There's lots of other minor errors, like components with the wrong value, swapped controls ("high" is in fact "low"), but the OPT is spot on, so the most expensive piece of kit is solid and will stay.
The tech is really taking a shine to this, and is putting in lots of time and effort. Will let you know when I know more.

[Mr. Foxen]

[quote name='razze06' post='1212466' date='Apr 27 2011, 11:44 AM']Looks like this is an extra complex job! apparently, the power stage is based on the bassman 100 schematics, but the pre is more like a fender twin. There's lots of other minor errors, like components with the wrong value, swapped controls ("high" is in fact "low"), but the OPT is spot on, so the most expensive piece of kit is solid and will stay.
The tech is really taking a shine to this, and is putting in lots of time and effort. Will let you know when I know more.[/quote]

Good when a tech gets enthused about an amp. It is what I always look out for.

[razze06]

[quote name='Mr. Foxen' post='1212478' date='Apr 27 2011, 11:54 AM']Good when a tech gets enthused about an amp. It is what I always look out for.[/quote]

I took another amp to him for a service, and he spent half an hour showing me the bits that were wrong with the valve amp, how and why, in much detail.
Fortunately I have some understanding of these things (not enough to do it myself), so i could appreciate what he was saying.
Good stuff, he said he wanted to get to the bottom of it, as he thinks this particular amp has annoyed musicians since 1974

[razze06]

One year on, I'm picking up the amp again from the tech in a few days' time.

He made several fixes (improvements), checked that the power stage is just fine, and can't do any more.

Since I brought it in, i've sold my old school cabs, and generally didn't really miss the amp. Hence, i'm thinking of selling it. It's been suggested that it may be worth more in bits, as it's made up of all the best components you could buy in 1974, and all are in good working conditions.

Any ideas or suggestions?

[Mr. Foxen]

Be really sure you want rid before selling it. It hasn't huge value as is, and replacing it will be a case of paying for an amp with some sort of name, because you are unlikely to find another like it without a name.

[razze06]

[quote name='Mr. Foxen' timestamp='1332433986' post='1588361']
Be really sure you want rid before selling it. It hasn't huge value as is, and replacing it will be a case of paying for an amp with some sort of name, because you are unlikely to find another like it without a name.
[/quote]

I'll have to try it again before I make any final decision, and I'm well aware that selling it won't buy me a real bassman or any other valve amp!

Then again, i've not had it for a year and gigged equally well with my other amps...Decisions

[Beer of the Bass]

Is the output up to scratch now? I reckon you should try it with your band before putting it up for sale. A healthy 100 watt valve head makes for a pretty good pub gig amp and ought to pair up nicely with your two 1x12" cabs if the impedance is right.

[razze06]

Well I got it back from the tech, took a few pics of the inside. Looks like all top quality components, dated from late 1973 to early 1974. Transformers are all in good nick, original Fender parts from the late 1973.
The verdict is that the power stage is effectively a clone of a Bassman 100, down to the components. The pre stage is a lot messier, possibly starting as a clone of a fender twin pre, and then "improved" over the years, probably by trial and error judging from the amount of corrections.
[attachment=103654:IMAG0015.jpg]
[attachment=103655:IMAG0012.jpg]
[attachment=103656:IMAG0013.jpg]
[attachment=103657:IMAG0014.jpg]

My tech's opinion is that it would have been easier to rip out the entire pre and rebuild as a bassman using the same components whereever possible (or as a proper fender twin pre). It's a lot easier than trying to second guess the intentions of the original builder 37 years ago...

I took it straight to band practice last night. Used it through a hartke 4x10 with a squier cv jazz. It actually sounds very good, and one of the channels is a lot louder than the others, so there it went

Extremely nice highs in an old school sort of way, the jazz truly sang when we did a bit of drum and bass/funk/dub jam. Volume wise, it kept up easily, but don't know how much clean(ish) headroom i had left. The volume knob was only about halfway up, but that means very little. The tech tested it and could rate it at around 40W clean, as the pre make it distort quite heavily pretty quickly.

Boy is it heavy...Make the Peavey Mark III feel light and positively portable...

[umph]

40w clean sounds really bad. theres no way you'd only get that much in fixed bias without there being problems elsewhere

[Beer of the Bass]

I guess the power stage could be working to spec but the pre-amp clipping early due to peculiar gain-staging. It may have been tweaked from a guitarists POV perhaps. Is that a PCB based build? Quite surprising for a DIY job - I wonder if the boards came in kit form at the time.

[umph]

the preamp would have to be really badly designed to clip that early. Also the tech could've dropped some lower mu valves in the pre? I just think he was being lazy.

[razze06]

I've tried the amp again at home last night. The pre doesn't clip almost at all, and the sound is clean(ish) all the way to max gain in the pre. That is with very low volume from the power stage, haven't tested the overall effect yet.
Interestingly enough, we tried bridging the inputs like to engage all channels, and we could hear a weird and interesting tremolo/flanging effect on the signal

The amp almost certainly has parts that started life as PCB bits from a kit, but there is plenty of signs of modification on the PCBs themselves. It is quite likely that it has been modded to alter the tone to suit a certain style of guitar playing. I had to ask the tech to stop working on it, as I wanted to make a decision about what to do with it. I still haven't...

The options I have are:
[list]
[*]Sell it as is, for around £200. Probably more appealing as a guitar amp, but the current tone is very nice for bass too. Tempting, but a bit of a wasted opportunity, as Mr Foxen says
[*]Sell the components separately. Original Fender OPTs from 1973 should fetch a fair bit of money, maybe £300/£350 for the lot? Does anyone else think that is close to murder ?
[*]Rip out the existing preamp, and have it rebuilt as a proper bassman pre (or even a fender twin pre) using as many of the existing bits as possible.
[*]Bypass the preamp, and use it as a valve power amp with another pre
[*]Keep it as is.
[/list]
What would you do?

[umph]

[quote name='razze06' timestamp='1333099779' post='1597265']
I've tried the amp again at home last night. The pre doesn't clip almost at all, and the sound is clean(ish) all the way to max gain in the pre. That is with very low volume from the power stage, haven't tested the overall effect yet.
Interestingly enough, we tried bridging the inputs like to engage all channels, and we could hear a weird and interesting tremolo/flanging effect on the signal

The amp almost certainly has parts that started life as PCB bits from a kit, but there is plenty of signs of modification on the PCBs themselves. It is quite likely that it has been modded to alter the tone to suit a certain style of guitar playing. I had to ask the tech to stop working on it, as I wanted to make a decision about what to do with it. I still haven't...

The options I have are:[list]
[*]Sell it as is, for around £200. Probably more appealing as a guitar amp, but the current tone is very nice for bass too. Tempting, but a bit of a wasted opportunity, as Mr Foxen says
[*]Sell the components separately. Original Fender OPTs from 1973 should fetch a fair bit of money, maybe £300/£350 for the lot? Does anyone else think that is close to murder ?
[*]Rip out the existing preamp, and have it rebuilt as a proper bassman pre (or even a fender twin pre) using as many of the existing bits as possible.
[*]Bypass the preamp, and use it as a valve power amp with another pre
[*]Keep it as is.
[/list]
What would you do?
[/quote]

the trem effect is the amp going into oscillation.

personally i'd redo the pre, since you'll be lucky to get 200 for it and the transformers aren't really gonna be worth as much as you think they are.

[razze06]

[quote name='umph' timestamp='1333108175' post='1597488']
the trem effect is the amp going into oscillation.

personally i'd redo the pre, since you'll be lucky to get 200 for it and the transformers aren't really gonna be worth as much as you think they are.
[/quote]

I am also thinking that redoing the pre is looking increasingly like the best option. The quotes are whatever the tech gave me. I am reasonably knowledgeable in matters electrical, but not in the value of vintage components