Hot Rod Deluxe bias issue (Justin???)

[bscepter]

I posted this in a couple of other forums (fora?) to hopefully get an answer:

I have a Hot Rod Deluxe. Recently, I put in a pair of Sino KT66 tubes and biased them at the bias test point at around 62 mA for the pair. I just received a pair of Groove Tube KT66HPs and put those in. With the trim pot in the middle (around where I was for the other pair), the HPs are reading 120 mA! In fact, the coldest I can get them is around 87 mA for the pair.

Thanks in advance!

According to Justin's invaluable HRDx site:

Q: The lowest I can bias is (a very high number). What's wrong?
A: The vast majority of people who experience this have really conductive power tubes. I'd return them and ask for a pair that doesn't conduct as well. If not I would check the screen resistors. Measure the voltage drop across the resistors, then calculate screen current. If the screen resistors seem fine and other tubes still conduct too well (100+mV read at the bias test point), then I would increase the negative voltage applied to the power tube's control grids by raising R77 in increments. This will solve the problem if the previous two do not.

So... what do "highly conductive" tubes mean to the amp? I played it for a few seconds, and the amp sounded great. Can they damage the PT?

[bscepter]

I think I'll return them for a lower breakup (less "conductive") tube.

However, according to Justin's bias calculator, figuring the HRDx's plate voltage at 430v and the max plate dissipation of the KT66 at 30w, the bias test point shouldn't exceed 97.67 mV. So... if I run them at 85 mV (which is the lowest I can go), am I still in danger of melting my PT? Or will I just shorten the life of the tubes?

[ragtweed]

The plate voltage will change depending upon the bias setting. If you're really pulling that much juice through those KT66's I'd bet that the plate voltage is below 430 in the HRDx. You're still not in the clear though. You have to calculate the dissipation, which means checking the bias and the plate voltage. If the output tubes in the HRDx are dissipating too much it could be a problem since the HRDx was designed to be a 40 watt amp. If the power transformer doesn't get too hot with the Sino's I would stick with them. 68ma at the bias test point should be okay. I wouldn't go higher than about 70ma myself.

[ragtweed]

I was just checkin' specs for the KT66 and they're rated for 25 watts of plate dissipation and 3.5 for the screen. Based on a nominal plate voltage of 430VDC biasing between 29ma - 42ma (per tube) would have the KT66's running in an acceptable dissipation range, but since the HRDx has components that are based on a 40 watt design figure I still wouldn't go higher than 35ma per tube (@ 430VDC).

FWIW: The KT66 draws 1.27 amps of heater current compared to .9 for a 6L6 type tube so make sure the P/T doesn't overheat. In my own HRDx I've lowered the plate voltage with a power resistor in the B+ line between the rectifier and filter caps. I use either 5881's or 6L6WGB's and I set my bias so that the overall RMS output of the amp is somewhere around 30 watts. I've never tried KT66's in my HRDx, but they work great in my BiValve.

[bscepter]

Hmm. I thought the "absolute max" plate dissipation on the KT66 was 30w (at least according to the Marconi-Osram G.E.C. chart).

I think you're right, though. I'm going to send them back and exchange them for a pair rated around 4 on the GT scale. I should be able to bias these around 60 mA. Either that, or I'll have to replace the bias trim pot with something that has a wider range. That would involve taking the whole bloody PCB off, which I don't want to do.

[RMS]

Some times it takes more than 50k (ie. trim pot) to re-bias an amp. You might be able to do it without pulling the board by just cliping out the bias resitor leaving some of the lead in place, then soldering a new resistor to that little bit of lead. Solder it quickly as to not melt off the solder on the side that you cant get to.

[bscepter]

Some times it takes more than 50k (ie. trim pot) to re-bias an amp. You might be able to do it without pulling the board by just cliping out the bias resitor leaving some of the lead in place, then soldering a new resistor to that little bit of lead. Solder it quickly as to not melt off the solder on the side that you cant get to.

Thanks. Sounds tricky, though. How do I anchor the new resistor to that tiny piece of lead for soldering?

[bscepter]

I just spoke with Myles Rose at Groove Tubes. He said that running them at 87 mA for the pair should be no problem. In fact, he said, this is still in the "cool" range for the tube. He also said that the HRDx power transformer should handle the heater draw with ease (he claimed they could handle up to 4 amps (!)). I'll try them out and report back.

I just hope I don't see smoke in the middle of a gig... (Though that would give me an excuse to try out our other guitar player's backup amp, a Marshall Bluesbreaker combo...)

[xkilo]

I had the same results with a pair of KT66s. The bias at around mid point was 120, with 397V measured at the plate. The amp sounds great, but I do have some concerns running that warm. The bias adjustment range with these tubes is 100 - 150ma. Anyone else have experience using these tubes? What is "normal"?

[RMS]

Thanks. Sounds tricky, though. How do I anchor the new resistor to that tiny piece of lead for soldering?

I just loop the new resistor around and squeeze then do a quick dab of solder enough to get a wet joint but not so hot that the solder on the underside would drip out; so it helps to clean the resistor leads before hand.

I think I'll return them for a lower breakup (less "conductive") tube.

However, according to Justin's bias calculator, figuring the HRDx's plate voltage at 430v and the max plate dissipation of the KT66 at 30w, the bias test point shouldn't exceed 97.67 mV. So... if I run them at 85 mV (which is the lowest I can go), am I still in danger of melting my PT? Or will I just shorten the life of the tubes?

A typical 6L6 has a 30 watt rating with 430V at the plates, my figure is 69.8 mA, if using a safe buffer of 70% max dissipation then that figure is 49 mA. So it all depends on the wattage of the tube and the plate voltage, so if you like the sound of tubes biased hot and you can afford replacing tubes more often then thats a personal decision.

[Kap'n]

A typical 6L6 has a 30 watt rating with 430V at the plates

That would be a 6L6GC. And the voltage doesn't matter - the plate dissipation rating is 30 watts, regardless. Overvoltage can cause other problems, but it's not plate dissipation.

A 6L6 (metal tube), 6L6G or 6L6GB has a plate dissipation rating of 19 Watts.

A 6L6WGB or 6881 has a plate dissipation rating of either 23 or 26 watts, depending on manufacturer.


All of those ratings became irrelevant when tubes stopped being manufactured in the West.

[wingnut1]

Don't the KT66 have a different output impedence than the 6L6? Isn't it going to be looking for 4 Ohms instead of the 8 Ohms?

[Kap'n]

Don't the KT66 have a different output impedance than the 6L6?

Tubes don't have "output impedances" per se. They have a range of impedance load values that accentuate different tube characteristics.

They're measured in kohms.

"4 ohm" and "8 ohm" are speaker impedances. The output transformer acts as a step down to match the desired loads.

[RMS]

That would be a 6L6GC. And the voltage doesn't matter - the plate dissipation rating is 30 watts, regardless. Overvoltage can cause other problems, but it's not plate dissipation.

A 6L6 (metal tube), 6L6G or 6L6GB has a plate dissipation rating of 19 Watts.

A 6L6WGB or 6881 has a plate dissipation rating of either 23 or 26 watts, depending on manufacturer.


All of those ratings became irrelevant when tubes stopped being manufactured in the West.

Do you always quote people on a half sentence?

[Kap'n]

Do you always quote people

Only when it's the only pertinent part.

Sorry. I've got an intolerance for half-truths on amp stuff.

[RMS]

Sorry.

Thats better.

There was no half truth in what I said. If you want to be a picky A*s or anal about a missed punctuated sentence and take things out of context then I have an intolerance for you.

How do you like being miss quoted now? It can be made to take things out of context such as you did.

[Kap'n]

There was half truth in what I said.

That would be correct. Learn electronics before you spout.

[yankeerob]

Msg for bscepter - I've been messing around with a HRDv for a while now and personally think you're getting in at the deep end by putting KT66's in your HRDx only in that what you're trying to do is outside the remit of the stock circuit of the amp, will cause more heat than already occurs and the one thing the PCB's in these amps do not like is heat!!!. Modding it is fine as long as you're prepared to accept the consequences - which in the case of the HR series - is the possibility of having to do far more than you might have expected as the PCB's in these amps are extremely fragile.

I thought because I've had the experience of modding lots of different electronic devices in my 20+ yrs experience as a prototype wireman I would be able to handle the task no probs but even with a temp controlled iron and the greatest of care I've had to 're-hard-wire' quite a few tracks around the mods I've done. It's not the fault of the mods themselves - the amp sounds better for them. I just cannot stress enough how easy it is to lift the pads on these particular PCB's as the copper is unbelievably thin and sometimes you cannot even see that the circuit has been broken - it's as if the copper has burnt or melted and shrunk back from the component leg (it's like that bit in the Wizard of Oz when Dorothy removes the ruby slippers from the wicked witch!!!). Thinking that you can avoid problems by limiting your work to the component side of the board is pure folly - and I don't mean that to dis any suggestions that anyone has made in this thread about doing so - I mean it from the experience of working on what is basically the same amp as the HRDx and HRDv use the same PCB's - just different PT and supply circuitry to support 60W output and a 60W OT.

Also - having had a look at the spacing between the power tubes - those KT's are gonna be real close to each other - I mean close enough to be thermally conductive let alone anything else... it must get really hot back there... I can't even imagine how hot they'd get on a gig!!!

Why KT66's anyway - I mean they look cool but - you're attempting to use a tube that the amp was way not designed for - in more ways than one