EL34 ----> 6l6 'mod' in a Hughes & Kettner

[mfin]

Hi all...

Thought I'd post this for some advice/thoughts after some great feedback that helped me on another thread...

I've had my Hughes & Kettner Tube 50 Head for years and years now and since getting it I've always loved it. But there's one thing I've always missed, and thats the 'punch in the guts' clean authority that you get out of a Fender Twin. My friends H&K TriAmp MkI (6L6) has that 'clean thing' too.

So... I sort of always thought that it would be great if mine was 6L6 loaded not EL34, based on my assumption (correct? wrong? stupid? other?) that plenty of 6L6 amps have that 'clean thing' going on and the clean sound 'as is' on my EL34 Tube 50 is weak and thin and disappointing (dirty though its just plain fantastic for what I want).

Well, a while back I noticed on the Tube 50 schematic that two resistors (to power valve cathode I think - but that could be rubbish, I'm far from expert enough) had written next to them two separate values, one for if youre using EL34's and one for 6L6's. Well, Kettner never offered this amp 6L6 equipped themselves but there you go, it seems possible.

I gave an H&K service centre a call and was told 'yep, change the resistors, plug the valves in, but then make sure you re-bias the valves' - in fact if you have a look at the schematic, (here it is http://www.gimme5.co.uk/kettner/Tube50.pdf ) on page 4, then you'll even see the valves are labeled 6l6GC (even though it never shipped with them), the resistors to the right of them are labeled with the two values.

So... I have the valves, I have the resistors... but I'm left thinking... firstly, will it change the sound like I imagine, cos I've convinced myself it might (but might change the dirty channel in a way I don't like for all I know).

Secondly, am I safe in changing the the resistors and valves and then powering up? I've got a stack of fuses. But once it gets to re-biasing, is it going to be miles out? is that possible? I've got a perfectly matched pair of valves. Reason I ask is that I've rebiased a Fender Twin cos it has easy 'ports' on the back for a multimeter and the pots and instructions are there for you. On the H&K I can see a tiny pot inside on the PCB close to the valves but its not like theyve made it 'possible for anyone' I'd have to get some simple instruction as to where to put my multimeter and what readings to obtain, checking that I've also found the right pot. OR... am I talking rubbish again????

Anyone got any pointers or thoughts on this?

Cheers

Mark

[MWJB]

If you're referring to the 1K 5W EL34 screen grid resistors (normally 470ohm in 6L6 amps) you do not NEED to change these for 6L6 operation. If you leave the 1K 5W where they are you can swap back and forth between EL34 & 6L6 with just a bias adjust (30-35mA per tube). If you change these resistors, then you will need to change them back if using EL34 again.

The easiest and safest way for you to check your bias is to use bias probes like Weber bias rite or TAD biasmaster, look expensive initially but after a few tube swaps without paying a tech, they have paid you back. Set negative voltage at pin 5 to maximum dc voltage (-55 to -60v?) before installing new power tubes, to be on the safe side.

Will the amp suddenly sound like a twin? No, it'll sound like a H&K with 6L6s. A twin has big trannies (so does the TRI amp) and a 12AT7 (ECC81) PI tube. Tolex twins don't have the cathode follower tone stack that you do, but tweed 80W twins do. Neither tolex nor tweed wins have the OP amps that the H&K has.

By all means try the 6L6s, may get you closer to what you want but it won't be "instant Fender Twin".

[mfin]

Well, I went for it cos I was itching to do it and I did it with a resistor swap to 470s like you said and as per diagram. I powered it up for a quick check and its working fine, its a bit more punchy and sparkly, but I didn't give it much of a run, just plugged it into a 1 x 12 i have lying around for practise and didn't wanna annoy the neighbours.

Don't suppose you can give me a more of an idiots guide to check the bias... I can see the pot labelled TR1 (see schematic) that must be for it on the board, but I really need to know which number pin I'm looking for on the valve, and do I measure voltage from that to an earth? Also, there's only one pot, this TR1 for adjustment??? Any more pointers?? I have a good multimeter here.

[mfin]

By the way, you commented on the big transistors (you mean the output ones, like you get in any amp yeah?) that a twin has, do people 'mod' their amps by changing these ever?

I also noted that Twins have a 12AT7 on each channel, would swapping one of my 12ax7 valves to one of these have any great effect?

[MWJB]

TR1 is the bias adjust pot, don't adjust it until you have a means of actually measuring your plate current. Bias is always measured by plate current...the easiest way to do this in your amp is with bia probes, they fit between tube & socket & have wires tyerminating in banana plugs, or their own meter. If you are going to be changing tubes in a fixed bias amp you should invest in one of these devices.

Readings like "-52v @ pin 5" on their own are useless, the reason I suggested upping the negative voltage was to allow least amount of current (more negative volts = less plate current) at turn on & hopefully not damage any tubes - bias can then be dialled in to taste.

There is another method by which you can measure plate current, called shunting the transformer. It doesn't really qualify as something to be incorporated in "an idiot's guide" as it means connecting the meter to the highest voltage present in the amp & is therefore relatively dangerous. It should only be attempted if you have a meter with clips/wire grabbers and if you make all necessary connections with the amp unplugged from the wall. All connections should be double checked before powering up, mistakes here can lead to blown meters/fuses/tubes/transformers, fires & ...oh yes, death.

DON'T TRY IT UNLESS YOU ARE GOING TO FOLLOW INSTRUCTIONS EXACTLY! IF THE INSTRUCTIONS DON'T MAKE SENSE TO YOU, GIVE UP & BUY SOME PROBES.

Set meter to dc current <200mA, connect red meter lead clip to "plug 17", connect black meter lead clip to "plug 3" or "plug 4" (these connect to the transformer primaries that connect to the plate wires at pin 3 of your power tubes) depending on which tube you want to read, you will want to check them both to ensure tubes are matched to +/- 5mA. Power down amp & unplug from wall whilst changing connections.

Power up, wait 30 seconds, throw standby...

If current rises above 40mA aftter throwing the standby, turn off amp, adjust TR1 and try again. 30-35mA would be good, 28-37mA would be OK if that's all the adjustment the pot will give.

After setting current set your meter back to volts/ohms or you'll blow it/fuse if trying to read a voltage whilst set to current...another good reason to use bias probes!

[MWJB]

I commented on "big transformers", yes people do change these but they are the most expensive components in your amp so you need to be sure that they fit, and that they have all the relevant taps/voltage supplies. In short, it would be cheaper to sell your amp & buy a twin.

Swapping the phase inverter tube to 12AT7 will clean up the tone some, this is the tube that feeds the power tubes (V3 in your amp).

[mfin]

Thanks for that.... yes, it does sound beyond me, but a good friend of mine repaired TVs, videos etc for 15 years so it'll make sense to him and he said he'd come round and give me a hand so we'll go through what you said.

I might get hold of a 12AT7 for valve 3 and give it a go, its not like its a lot of money.

As for the twin sound, I'm happier with my amp than I ever was with a red-knob twin so I'm only looking for what I might be able to improve/tune to my taste, nothing more, I'll try it through my 4 x 12 again and see how its changed from the re-valving.

I know what you mean about transformers, of course the downside of a Fender Twin or an H&K TriAmp is that they both weigh an absolute ton! I did wonder whether the 100 watt version of my amp would sound any different but I doubt that it would have much difference other than output volume and 'output possible before clean channel starts to break up'.