New Tim Mellows OTL project

[nafunga]

Hi all,
Thought I'd add my 'twopennyworth' to the posts on the use of a non-centre tapped HT transformer.I made a couple of these Amps. one in Dec.2010 (6C41C) and the second (6C33C) in early 2011. (Both builds were posted on this site) Both are working well with no problems). As mentioned in my previous posts, I used ex-TV workshop Bench Isolation transformers (500W) in both designs. The only problem with this is that, because of the use of DC coupling,it is possible, that due to valve ageing or component drift (due to heat) the two halves of the channel become unbalanced. This may not show on the ammeter as this should show the current through the combined output stage - but, it may unbalance the 150v + & - rails, causing one rail to increase whilst the other decreases proportionally - this could of course cause damage to the ht smoothing capacitors particularly as they may not be rated for the combined ht voltage possible under these conditions. A simple insurance mod (!) (which I've done to both my amps, is to fit a 0-300v Voltmeter across one supply rail to ground. (Does,'t matter which) This then gives a visible indicatiion of any drifting of the ht voltages. The meter should normally read around half of the full rectified ht (300v or so). As a bonus, it makes balancing of the circuits vey easy as, with one stereo channel backed off (minimum current on the ammeter) the other channel balance can be set noting any movement from the correct voltage on the voltmeter. Then the standing current can be easily set. This channel is then backed off to enable the same proceedure to be used for the other channel.
Hope this makes sense (!) Having used this mod for a while it does make me feel happier that I should at least get an indication should unseen drifting occur.

Regards,

David (Nafunga)

[smith01793]

Hi David,

Thanks for the info
Sorry for not posting reply before now but I have been working over the last few days and have not had chance.

Working on the amp yesterday I have done a test of the voltages without any tubes and I have found imbalance on the +150 and -150 rails (about 30v)
To try to pin down the problem I disconnected the feeds to the 146v and -430v and with just the main caps connected to the bridge rectifier, the centre voltage is spot on. If I then reconnect the AC to C19 for the -430v the problem comes back.
I have double checked all the connections in the -430 rail and everything is ok

have you checked the voltage on your amp with no tubes?
if so do you get an offset ?

When I fit the pre/driver tubes I get correct and equal anode voltages on v1..
do you think when the output tubes are in and conducting the voltages will stabilise?

any help or comments gratefully received

Tony

[nafunga]

Hi Tony,
Yes, that's perfectly normal, as the current drawn by the -ve supply (even with no valves) is enough to un-balance the 150-0-150 ht rails. The ht to the input (12AX7) is of course drawn from the + 150v rail and tends to offset (and balance) the -ve rail current. It's only when the valves (6C33's especially) age that there is the possibility of the HT inbalance becoming serious enough to cause problems - hence the addition of the voltmeter.
Hope this helps.

Regards,

David.

[smith01793]

thanks for that,

having analysed the circuit I can see that there are two effective capacitive dividers in play here, +150/-150 and +146/-430v and both are centred around the earth point, the latter pair pulls down the centre point giving a higher +150v rail,

would this still be the case using a centre taped xformer? In Tims circuit there is a 1k in series with the CT surly this will allow the CT voltage to float anyway ? ?

Tony.

[nafunga]

Hi Tony,
I would expect that fitting the C.T transformer would help to overcome this problem, but agree that the 1K resistor might compromise this. Maybe someone who has built using a CT transformer could enlighten us?

Regards,

David.

[smith01793]

Hi,

Had a brainwave today while driving home from work!

I have spare a 500va multi tap auto transformer in my spares stash,
this is tapped at 0 - 100 - 110 - 115 - 220 - 240

I can use this to create a centre tap on my isolating transformer.

As the mains voltage in my area is reading 250v, I have fed in to the 0 - 240 tap from the isolating transformer and then used 0 - 100 -200
(effectively giving me 105 -0 -105) the centre tap is feeding through a 680 ohm resistor, as this is the closest w/w in my box to 1k.
This has stabilised the + and - rails to within 10 volts with the DC of-sett at 0.2mV per channel

this is giving me a total ht of 285v before it was about 340 Volts (too high?)

I tried shorting the centre tap to ground directly and this of course split the ht dead centre but it produced an audible hum (maybe because it effected the off-set) but I didn't leave it on for long enough to do any other tests

I have run the amp for 5 or 6 hours now and I am keeping the bias to around 100ma at lowish volume..

WOW!

I have been listening to and playing with audio for almost 50 years and this is the best sounding amp I have ever owned.

I have Monitor Audio Silver 6s speakers , I bought them for their overall smooth sound, but.. they have always suffered from a slightly indistinct "woolly" bass sound.... not any more, even at moderate volume the bass is distinct and clear!

the sound stage is also more distinct its like listening on headphones , the voices seem so real.

I intend to run in at low bias for several more hours before turning it up any more but I cant wait to hear it loud,

perhaps I should finish the enclosure first (so that I don't electrocute my family or myself)

See picure.

Tony.

[M Gregg]

Just a thought,

How hot is the wall behind the 6C33C's?

Remember that you have an open chassis at the moment as soon as you enclose it think ventilation..

Regards
M. Gregg

[smith01793]

Hi,

The wall is 7" away from the tubes and is only just a bit warm, as for ventilation here is a picture of the mock up of my design.

The chassis plate is quite cool thanks to the stand off's on the tube bases plus there is only 1 transformer underneath(20va for the driver tube heaters).

I may add fans below to cool the tube bases, I will see how hot it gets when finished.

Tony.

[M Gregg]

Originally Posted by smith01793

Hi,

The wall is 7" away from the tubes and is only just a bit warm, as for ventilation here is a picture of the mock up of my design.

The chassis plate is quite cool thanks to the stand off's on the tube bases plus there is only 1 transformer underneath(20va for the driver tube heaters).

I may add fans below to cool the tube bases, I will see how hot it gets when finished.

Tony.

Interesting,

Use of the strip to create the structure..Very inventive

Its a shame you can't enclose the Tx's at the back. I guess you could use a structure mounted again on the all thread at each side..

I guess it should be Ok without the fans there is plenty of cooling with the "finned" enclosure.

Looking forward to the pics of the completed unit..

I must admit I have to fight the temptation to put a few bits on the tubes and create Daleks.LOL

Regards
M. Gregg

[wicked1]

Like M.Gregg said, that chassis is a great idea!!!

I'm planning one of these too.. Was even thinking parallel output tubes, but seeing all those transformers on the back for the filaments has brought me back to reality..... I'll stick w/ 4 6c33


Now stick some dalek eye's on the tubes and call it Davros.