JapanAxe Posted December 4, 2013 Share Posted December 4, 2013 (edited) This amp belonged to a bass player who had retired from gigging, my friend bought it from him out of pity (along with a 2x12 cab from which he kept the drivers), then asked me if I'd like it. Having replace his arm in its socket and reimbursed him the paltry sum which he had paid, it is now mine: This will be a bit of a project for me - learn the schematic, test the valves, replace the filter caps, replace the worn mains lead, plus any other necessary work. Gut shots to follow! Anyone else got one of these? Edited December 4, 2013 by JapanAxe Quote Link to comment Share on other sites More sharing options...
EssentialTension Posted December 4, 2013 Share Posted December 4, 2013 I haven't got one but I did have one between 1974 and 1975 Quote Link to comment Share on other sites More sharing options...
Wooks79 Posted December 4, 2013 Share Posted December 4, 2013 How's it sound? I'm tempted to pick one up. Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted December 4, 2013 Author Share Posted December 4, 2013 [quote name='Wooks79' timestamp='1386183596' post='2296902'] How's it sound? I'm tempted to pick one up. [/quote] At the moment - appalling! Pots are scratchy and intermittent. Even at living-room levels, horrendoulsly farty on both channels, almost like ring modulation. I have yet to look inside though... Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted December 4, 2013 Author Share Posted December 4, 2013 Pics! Rear panel: Inside rear of amp. Two Mullard EL34 valves. Mains transformer on left, OPT on right, and mahoosive choke hiding behind two dual 32uF caps: Cover off - now you can see the choke more clearly. Panel-mounted controls and pre-amp valves (phase splitter not shown): Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted December 5, 2013 Author Share Posted December 5, 2013 More internal shots: Rather worrying blackening of the insulation on the mains switch terminals: Happily, all the valves tested good - 5x unbranded ECC83 (12AX7), 2x Mullard EL34. Another slight concern - the HT fuse had previously blown, and been bypassed by some fine wire (fuse wire?) wrapped around the fuse. Bonkers! Quote Link to comment Share on other sites More sharing options...
EssentialTension Posted December 5, 2013 Share Posted December 5, 2013 [quote name='JapanAxe' timestamp='1386277157' post='2298268'] Another slight concern - the HT fuse had previously blown, and been bypassed by some fine wire (fuse wire?) wrapped around the fuse. Bonkers! [/quote] In the old days you'd be lucky if it was fuse wire because often it would be the silver paper of a sweet wrapper or a fag packet or similar. 1 Quote Link to comment Share on other sites More sharing options...
Roland Rock Posted December 6, 2013 Share Posted December 6, 2013 [quote name='EssentialTension' timestamp='1386279924' post='2298325'] In the old days you'd be lucky if it was fuse wire because often it would be the silver paper of a sweet wrapper or a fag packet or similar. [/quote] Hah, yeah, I've seen paper from a fag packet used. Mr Foxen recently posted a pic of an old guitar string being used too! Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted December 8, 2013 Author Share Posted December 8, 2013 Spoke to a friend who is a materials scientist specialising in polymers. The blackening of the insulation on the mains switch terminals could be PVC breakdown, where the plasticiser separates out (visible as little globules in this picture). If overheating had been the problem, the switch body itself would most likely have become discoloured. Good news then. Also just discovered what a pair of working Mullard EL34 valves go for on eBay - more than I paid for the amp! Quote Link to comment Share on other sites More sharing options...
yorks5stringer Posted December 8, 2013 Share Posted December 8, 2013 I had a Truvoice back in the late 70's that was the same inside a different cabinet: got rid of it for a nice new solid state head! Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted December 8, 2013 Author Share Posted December 8, 2013 [quote name='yorks5stringer' timestamp='1386519394' post='2300946'] I had a Truvoice back in the late 70's that was the same inside a different cabinet: got rid of it for a nice new solid state head! [/quote] Very sensible! Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted December 13, 2013 Author Share Posted December 13, 2013 New filter caps today. The 32uFx32uF are fine but the 100uF are too small in diameter, so sent them back to Hot Rox and paying extra to replace them with 50uFx50uF - I will wire each unit in parallel to give me 100uF. Quote Link to comment Share on other sites More sharing options...
discreet Posted December 21, 2013 Share Posted December 21, 2013 (edited) [quote name='JapanAxe' timestamp='1386501744' post='2300695'] ...just discovered what a pair of working Mullard EL34 valves go for on eBay - more than I paid for the amp! [/quote] Wow, I see what you mean... I've got an amp on its way with four of 'em in - I sincerely hope they're working OK. I assume you tested the valves with er... a valve tester..? Edited December 21, 2013 by discreet Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted December 22, 2013 Author Share Posted December 22, 2013 [quote name='discreet' timestamp='1387651930' post='2314286'] Wow, I see what you mean... I've got an amp on its way with four of 'em in - I sincerely hope they're working OK. I assume you tested the valves with er... a valve tester..? [/quote] Yes I have an Orange valve tester - quite an expense, but it's already had a lot of use. I own 6 amps with valves in, plus it has come in handy for my friends' amps. Quote Link to comment Share on other sites More sharing options...
discreet Posted December 22, 2013 Share Posted December 22, 2013 [quote name='JapanAxe' timestamp='1387671662' post='2314498'] Yes I have an Orange valve tester - quite an expense, but it's already had a lot of use. [/quote] So... have you considered starting a postal valve-testing service..? Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted December 22, 2013 Author Share Posted December 22, 2013 [quote name='discreet' timestamp='1387706267' post='2314642'] So... have you considered starting a postal valve-testing service..? [/quote] In most cases it would be more cost-effective to buy a new valve! Quote Link to comment Share on other sites More sharing options...
discreet Posted December 22, 2013 Share Posted December 22, 2013 [quote name='JapanAxe' timestamp='1387728409' post='2314943'] In most cases it would be more cost-effective to buy a new valve! [/quote] I'm sure it would. I'll have to look out for a tester on the Bay. Quote Link to comment Share on other sites More sharing options...
4 Strings Posted December 23, 2013 Share Posted December 23, 2013 Does it have the shared grid resistor? If so, will you be making the mod to put one on each valve? And if so again, can you tell us what you do, how you did it and how it went? (If you've got some old Mullards in you won't need to, only if you're replacing with modern valves.) Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted December 23, 2013 Author Share Posted December 23, 2013 [quote name='4 Strings' timestamp='1387758788' post='2315423'] Does it have the shared grid resistor? If so, will you be making the mod to put one on each valve? And if so again, can you tell us what you do, how you did it and how it went? (If you've got some old Mullards in you won't need to, only if you're replacing with modern valves.) [/quote] I've been in touch with Mr Foxen and he says the 50W version doesn't need the mod, only the 100W version is affected. If at some stage I need to change the output valves, then I will see if it becomes a problem. Quote Link to comment Share on other sites More sharing options...
Sibob Posted December 23, 2013 Share Posted December 23, 2013 A guitarist friend has 3 of the older model Treble N Basses, loves them, runs them with the 1x15 cabs they came with . 50w may or may not be enough for you to gig with, but by god you'll have a sweet recording amp! Si Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted December 23, 2013 Author Share Posted December 23, 2013 [quote name='Sibob' timestamp='1387819705' post='2316105'] A guitarist friend has 3 of the older model Treble N Basses, loves them, runs them with the 1x15 cabs they came with . 50w may or may not be enough for you to gig with, but by god you'll have a sweet recording amp! Si [/quote] I reckon it will be pretty loud through a BF or two! Quote Link to comment Share on other sites More sharing options...
Sibob Posted December 24, 2013 Share Posted December 24, 2013 I reckon it'll be great, but might break up fairly early? Which might be suitable to you Si Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted March 14, 2014 Author Share Posted March 14, 2014 Nearly three months on and I have (1) resolved the problems sourcing the right caps, (2) finally secured an uninterrupted couple of days to work through the fixes on this amp - and it's up and running! I have replaced all the filter caps, plus the electrolytic in the EL34 bias circuit. I have noticed there are 3 more 22uF electrolytics in the 12AX7 bias circuits, but they don't seem to be causing any bother. I'll probably replace them too though. There's still some scratchiness and loss of contact on the Normal channel's treble pot, but I have just located an exact replacement (CTS 100k LIN) in my bits box. Otherwise I can report that the amp is bloody loud, yet fairly easy to drive into distortion using my PS Mustang. Initially I wrote off the Bass channel as being - well, too bassy; then I jumpered it across to the spare input on the normal channel, and hey presto, mix and match! Here's the new filter caps in place: And here is the mains switch, now free of nasty chemical deposits. I have fitted an IEC male connector on a short trailing lead for the time being. I need to work out the feasibility of fitting an IEC inlet socket (problems: 1.6mm sheet steel, restricted space between mains transformer and rear of chassis). Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted March 14, 2014 Author Share Posted March 14, 2014 Treble pot replaced and all is well! I shall be encouraging Mrs Axe to indulge in some (restrained) shopping tomorrow morning while I fire up this baby through: (1) Laney 2x10 for guitar mayhem; (2) BF Compact and Midget connected via OBBM serialiser (to give 16ohm load - no 4ohm tapping on transformer). Quote Link to comment Share on other sites More sharing options...
discreet Posted March 14, 2014 Share Posted March 14, 2014 Good work! I love seeing valve gear brought back to life. Quote Link to comment Share on other sites More sharing options...
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