Tube choices

[skyboltone]

Greetings:
I'm using an OT that will take to KT66 or 6V6 tubes.

I've gathered material for the KT66 version, including tubes and PT.

I will likely only be playing small venues, (church) I live on a 48' boat where the rule will be turned way down low, except when in I'm in the middle of the Bay.

Will a KT66 ODS clone work turned way down or should I give a longer look to 6V6 finals.

Most comments I've heard are that HAD amps relied mostly on the preamps for distortion. The preference would be KT66 because I'd like to get loud if I want, but I need to be realistic about my needs.

The speaker will be an Altec 417 series just like the early HAD amps.

Thanks in advance for opinions

Dan

[Bob-I]

In my experience 6V6's are not much lower in volume than 6L6's/KT66's.

Also, Dumbles really do sound good at low volumes. Like any amp they sound better when you can move some air with it, but I'm totally happy with the low volume sound of mine.

[Luthierwnc]

I agree with Bob-I, an amp sending this much signal into the power section is going to be pretty loud with 6V6's anyway. Part of the beauty of this design is that you can put some hair on it at most volumes.

Still, you really don't need to choose if the transformers are rated for the KT66s. They have the same pin-out so you can change them on fairly short notice. A couple of issues; the tubes have really different bias points so you'll need to have a switchable fixed-bias control. It can be a completely different set of bias components or you can hang another trim pot in place of the tail resistor (15k). Use the switch to short the wiper. I like the latter because either position always has some negative volts on the grids. You could also just run the 6V6's cathode-biased.

The other issue is the power tube draw will effect the voltages downstream. These amps are pretty touchy about that. If you have problems getting the preamp volts right, you could use the other half of the heavy duty DPDT switch mentioned in the paragraph above to add a series power resistor between the screens and the PI. I haven't ever done this so this is just theory.

Let us know how you do.

Skip

[skyboltone]

Thanks Bob and Skip:
I hadn't considered that they had the same pin outs. I bought a fancy Bournes 10K bias pot for this build but I have a snazzy 22K wire wound (yea!) ceramic pot that I'm dieing to use somewhere.

I've also got an old 100MA 2" meter, antique. If there's room I'll set it in the bias circuit to keep things easy, on the backside of course.

As far as the series resistor for sreen voltage manipulation, all of the literature I've seen recommends against it. Raises distortion products pretty bad.

Really looking forward to getting going. The main Mouser order has not been sent yet, but I do have the Westlabs and Antique Electronics stuff here.

KT-66s for now, with provisions for 6V6s later maybe.

Cheers
Dan

[Luthierwnc]

FWIW, in my build I used an Angela Universal PT which B+'s around 485. The conventional set of dropping resistors left me about 20 volts too high on the PI, V2 and V1 so I just used an 8k in place of the 3k between B+2 and B+3. Worked perfect. I look at those resistors as a base line. The preamp voltages are what you are shooting for. If you had a PT on the lower side, you might need to reduce those values.

Skip

[skyboltone]

FWIW, in my build I used an Angela Universal PT which B+'s around 485. The conventional set of dropping resistors left me about 20 volts too high on the PI, V2 and V1 so I just used an 8k in place of the 3k between B+2 and B+3. Worked perfect. I look at those resistors as a base line. The preamp voltages are what you are shooting for. If you had a PT on the lower side, you might need to reduce those values.

Skip

I hear you Skip. I've bought a whole series of 3 watt resistors for the line, just to be sure I've' got the right voltages to start. It has been my practice to get all the power supplys up and running before doing much else on transmitter and receiver projects in the past. More predictable outcomes that way.

I was reading, in an old power supply design paper about chokes in audio equipment. Seems that a completely unloaded choke will swing to infinite inductance when turned on with nothing but heating tubes to look in to. I'm planning on adding about 50k in a parallel bleeder tap (10ma) first thing right of the choke. This make any sense to you?

Must be hot and miserable everywhere, not much traffic on the site.

Dan

[Luthierwnc]

Uh, didn't quite get that. Can you describe that bleeder tab a little more?

sh

[skyboltone]

Ordinary practice has us placeing 270K - 470k across the first couple of capacitors in the power supply line. This places all of the bleeders ahead of the choke. The old paper I was reading describes putting something around 3%-5% of the power supply capacity in the form of a bleeder resistor just downstream from the choke. Apparently when the power supply is energized without any load on it, the choke can swing to infinite inductance and cause fuses to blow and be pretty hard on the rectifier and transformer.

!?!?

Dan