ebay score

[black_labb]

just won this 200watt tube bass amp on ebay.

http://cgi.ebay.com.au/ws/eBayISAPI....tab%3DWatching

its an Australian/New zealand designed and made amp from the late 60's/early 70's, which is apparently based quite heavily on hiwatt designs. havent found a schematic or anything more than 1 for sale from some vintage guitar/bass/amp shop. the amp runs on 4 6550 tubes, some of the little info i found is that it is similar to a hiwatt circuitwise. the thing also came with a matching bass cabinet.

the guy said in the ad that it was currently running on 2 6550's. i asked him wether its full number was 4, or something else and how many preamp tubes it has. he sent me back saying "there are 4 valves in it all in working condition, as for the rest im not sure" i dont know wether to take that as 4 poweramp valves or 4 in total (there are 2 channels, so i doubt that it is 4 in total) or wether he put another 2 in to make sure that they were still working and then said that there were 4 in there. he may have pulled the other 2 to not use them up as they are expensive tubes to replace.

aparently is very oringinal and good condition, but i got it thinking i might use it as a platform for modification. was thinking i may put a preamp with a master volume into it (theres a thread about it in the theory and design about overdriving a pentode pamp valve to get an idea for the pamp i was thinking of). might also keep it original and do a different amp thats specifically for guitar with that pamp.

the only issue is if the powertubes need replacing, its 300$ australian for the replacements being 6550's. if thats nessisary i may consider converting them to el34's (either 4, or add another 2 to get 6 and keep the 200 watt output. that will depend on the PT in there though, as im sure the 6550's run at much higher voltage than el34's would.

if anyone happens to know where i could find a schematic that would be great, but i heard that their amps were made with what they had in stock, so they might not be the same, but im sure things like the tone stack and things like that would be the same. just maybe different tubes.

[Steve Conner]

If it's 200W, it will have 4 6550 power tubes. $300 is too much for four Sovtek 6550s, even in Australian or NZ dollars, someone is having a laugh.

To get 200w from four, they will be running at something like 600-700V plate voltage. EL34s were originally rated up to 800V, IIRC, but I don't know if new production ones will take it.

As a last resort you could try the Sovtek 5881, also known as the 6Pi3S-E. I don't think you would get the full 200w from four of these, maybe about 130.

[black_labb]

If it's 200W, it will have 4 6550 power tubes. $300 is too much for four Sovtek 6550s, even in Australian or NZ dollars, someone is having a laugh.

To get 200w from four, they will be running at something like 600-700V plate voltage. EL34s were originally rated up to 800V, IIRC, but I don't know if new production ones will take it.

As a last resort you could try the Sovtek 5881, also known as the 6Pi3S-E. I don't think you would get the full 200w from four of these, maybe about 130.

just had a better look at the tube place, didnt see the EH 6550's which go for $230 for a quad, which is australian dollars. sounds better than the 300, but still not as cheap as retuing the little 5e3 ive nearly finished. found another supplier that carries sovtex 6550's, and they run at 190 for a quad, which is around the right markup for imported things in australa (smaller market means more expensive imports)

just had a look on tube depot to see what prices you guys were paying over there, and saw that the prices for sovtek was around 45-50, and thought that sounded about right for me to be paying 230, but then i realised that that was for a matched pair. might get some when im visiting family in canada/america for the christmas holidays.


edit: just found some more info on the amp (not this one in particular, but some of the companies designs which seem similar but using different/less tubes. seems that many of the circuits had a 12a*7 driving after the phase inverter, which is apparently fairly common in hifi amps. im looking forward to seeing this thing and hearing it. it would be great if he had a manual with a circuit diagram, but i doubt it.

[black_labb]

i went over to pick it up tonight, it powered on, but soon after flicking the standby switch something blew. were quite sure it was an output tube. the guy selling it was a good guy and wants to get the problem sorted before i pay for it/get it. well keep in touch to get he said the amp has been used twice in the last 20 years, one being earlier in the week to make sure it still works.

the amp has 4 6550's (he couldnt verify that they were 6550's in the email, as he only could see the numbers on 2 of the valves. has 4 preamp valves (they are 12ax7's atleast for 2 of them, didnt see the writing on the other 2).

it ended up being a guitar amp, for some reason i was under the impression it was a bass amp. i dont know why. im still happy to get it.

the speaker cab has 4 50w australian made speakers that were standard on the amp, look to be ceramic magnets. they also seem to be in great condition (the cab itself has some tears and such, but thats purely cosmetic)

the head is in great condition, one of the transformers is fairly rusty on the laminations, but it should be alright.

[black_labb]

the problem ended up being just a fuse, but i got a matched quad anyway.

the amp sounds great. channel 1 has a bit more gain giving it some crunch, and channel 2 has a great clean sound. the amp is nice and punchy (its 200w, it would be hard for it not to be) with a nice warm sound. its not particularly versatyle, but what it does it does well. nice crunchy lower level distortion, and a beautiful clean sound.
the components are pcb mounted, but being a simple circuit, the tracks are a good distance apart and easily serviceable. the preamp is actually in a different section of the amp which is interesting. the circuit uses 4 12ax7's and 4 6550's. 2 12ax7's are used in the preamp, 1 for PI and the 4th in a post PI gain stage (1 triode each side). the circuit seems to be in great condition.
the speakers are 12" alnico "plessey/rola 12u50" wich are australian made.

here are some pics for anyone interested.