Help - Valve Pre-amp problem

Folks - I have been experimenting with valve pre-amps recently and have been pleased with the results. I then found a Croft Vitale SC on ebay and won it, and it arrived yesterday.

All seemed fine, and I was quite pleased with the sound last night and looking forward to more listening today... I turned it off last night so turned it on again this morning and went away for 10 mins to have a coffee.

Came back, played some music..... horrible distortion on right hand channel, sound disappears if turn volume up (left and right channels have seperate volume controls).

Checked all connections, all Ok. Tried different inputs - line 2 instead of line 1 etc, no difference. checked my source into headphone amplifier - no problem. checked power amp via said headphone amp (it doubles as a preamp - again no problems)

Finally, opened pre-amp. It has 3 valves, 1 ECC81 and 2 ECC83s. I swapped the 2 ECC83's around to see what happened - no difference, problem is stilll in right hand channel.

I don't have another ECC81 to try, but is it worth getting one to try out? or is the problem elsewhere. Very odd that it was OK last night but problems this morning - and very disappointing.

Any ideas?

 

Well I know of the vitale a little bit, its very simple indeed, hard wired too, so should be fairly easy to fix.
Its possible its a dry joint happened during transit, no cure for this other than to check voltages and all that, and go over every joint with a soldering iron.
Otherwise, capacitors, or resistors, if you have swapped valves chances are its not those, tho' it could be the one on its own, so new valves all around too. ecc81's are pretty cheap anyway.ecc83 would work ok tho' I would think. If you can work out which valves do the phono stage, you can extract them and just use the line stage on its own, with all the valves, and see if the problem is still there, that way you will know if its the valves or not.
ecc83 is safe in ecc81 position. not a lot of difference between them.
There could be 250-350 volts inside this, so no touching if you don't know what you are doing, even if you do, I have had about 5x240 volt shocks, so care needed.
Got a valve preamp problem myself and can't seem to fix it, its a pcb, and has fets as regulators, which complicates things and so makes things quite tough. May have to get my bro on to it, he is cleverer that way than I.

 

I think swapping valves around is the limit of my ability.... will see what I can try.

 

do whatever is easier and/or cheaper - either get ecc81 or take to the "master".

 

you said problem went away when you turned up the volume??? try some contact cleaner on the volume pot. with switching tubes around etc, yield no results and problems persisting, i'd bet it was that. turning it up wouldn't cure a cold solder joint either.... try the pot.

 

No - it doesn't go away - the sound gradually fades totally on the right channel if the volume is turned up. I'll try it though.

 

don't know then.

 

Tried contact cleaner - no difference. Sprayed it into the volume pot and gave it a good waggle.

By process of elimination I have deduced that the line stage runs from the single ECC81 valve (how does a single valve do stereo)?
Replaced it with one of the ECC83s - no difference. Still distorted on right channel.

Oh bugger... I'm not going to be able to fix this without sending it to someone. The person who sold it was advertising it with all documentation receipts etc but they are not there. He says he's sending them on immediately.

Beats me why it was working fine yesterday. Guess the next step is to contact Croft.

However, I am beginning to feel my flirtation with valves is coming to an end.

 

it's a twin triode. my pre uses ecc82 which is similar. it sounds like it isn't the tube. i would hold judgement on the flirtation. contact croft i would be surprised if they couldn't offer assistance. see if you can trace the circuit path and see if anything looks out of place (damaged / loose). i don't know anything about croft gear, but tubes imho are fab.

 

take it to retro reproduction in edinburgh.

I doubt the repair will come to more than 30-50 quid absolute tops.

 

Sorry to hear that your new toy is fault. Yet another case of someone buying a faulty component on Ebay. I just dont trust it at all

 

Hi Alan

Don't be despair with your Croft pre amp. I have seen the internal of this amp and there are very few passive components which can go wrong. Any competent electrician or TV repair engineer who knows about valve amps can repair it.

Both ECC81 and ECC83 are double triode valve which means it can amplify two separate signals, i.e. stereo.

Good luck.

Regards
Hon

 

Well - its been a very long strange day.....

As a last throw of the dice I took everything apart, re-fitted all connections between components and between power amp and speakers, then powered up again...

The problem had gone. After half an hour assuring myself I was not hearing things (seperate volume controls for left and right is quite useful after all) it seems that the problem has vanished.... with no real explanation.

Then, after going away to do something else for half-hour, I came back and found distortion once again but now on the left hand channel. Its different from last time, sounds like listening to it through heavy rain. Whereas last time it was like listning through strong winds, if you can imagine the difference.

Ran through all process-of-elimination checks again - this time the power amplifier (PS Audio HCA-2) appears to be the cause of the problem. Sure enough, when I open it up to take a look, not expecting to find anything that I could diagnose, I see one of two glass 5A fast-blo fuses contains a very twisted and unhappy looking wire, close examination shows a tiny gap in it.

So hopefully I can find a replacement at Maplins tomorrow. And hopefully I'll not suffer any more gremlins. I'm still at a loss to explain the original problem with the pre-amp. Suggestions anyone?

Thanks for help ideas so far, with luck I'll be able to continue enjoying the croft once I get the power amp sorted.

 

see>!>! its not troublesome valve amps but pesky solid state that causes all the problems

what you need is a valve power amp

 

I suspect there is stil some kind of dodgy connection in the Croft pre-amp. And the power amp problem might have happened if I unplugged the pre-amp without switching the power amp off first, which might have happened at some point.

I have a Valve power amp - the Graaf 5050, have found it runs too hot for my domestic convenience, but will definitely give it a try with the Croft.

 

oh yup, forgot about the 5050. ummm nice.

I know you were worried about your kids, but IME they soon learn not to touch hot amps.

 

Fuse questions?

Regarding the power amp problem.... I got a pack of 10 glass 5A fast-blo fuses from maplins yesterday for the grand amount of 79 pence. I replaced both fuses in the power amp with said new ones.... its definitely fixed the problem and I'm listening to music again.

However, I can't help but feel the sound is not as refined as before, is slightly veiled and rough.... is there such a thing as an audiophile grade fuse I should be looking for? or do fuses have a burn-in time? Or am I imagining things?