dmcs414
100+ Head-Fier
So the verdict is, as I feared, kaput. Once I get a new amp, I'll see if anyone around here is interested in my CTH innards. Poor little guy.
IOW I'd give it a whirl, granted its lower risk for me because I'm a builder
So, after seeing a couple tubes were tried in CTH that had other than 6.3v & 12.6v heater and found to work & sound good, knew there was nothing keeping the design from directly supporting more heater voltages.
Nothing but a few more resistors & a way to switch among them.
Calculated (ideal) R3H values for various heater voltages:
Votage / R3H (ideal)
4.2v / 3.3k
5.6v / 5.8k
6.3v / 7.4k
7v / 9.3k
8.4v / 15.1k
9.4v / 23.3k
12.6v /
Wiatrob was willing to test & confirm things & has done so using 4BQ7A, 5BQ7A, 7AU7 and 9AU7 tubes. This tweak further extends tube options in the CTH beyond 6.3v & 12.6v heated tubes. So users of a CTH with this tweak have an even larger selection of tubes to choose from, probably finding some very good deals for tubes running at non-std heater voltages.
Edit: So beyond the 6.3/12.6 tube list, this tweak should add those above, 7DJ8 and 8FQ7 and there are likely more overlooked (e.g. 6AU7, 6BQ7)
The challenge is choosing which heater voltages you want & finding a SP#T switch to switch among them. We found a nice 1/2 inch dia one that can be configured for 2 -> 12 positions:
Wiatrob can chime in w/more info, but I think it remains to be seen if this tweak can be squeezed into the BoM case.
Here's a dumb question!
How critical are these heater resistor values?
I could only source the following 1/8W values:
For 5.8k > 5.828k? close enough ?
" 7.4k > 7.39k ? " " "
" 9.3k > 9.31k ? " " "
" 15.1k > 15k or 15.2k ? close enough ?
" 23.3k > 23.2k or 23.4k ? close enough ?
How close is close enough? Is it better to go up, down or simply the closest value?
I realize I can go 1/4W and/or add resistors but I don't want to piggy-back resistors to tweek the values as I'm trying to keep it Compact with the heater switch. Would like to stay with small 1/8W resistors.
It is not a dumb question. The values you've sourced may be the closest to the "calculated" values I came up with (& probably rounded to 0.1k).
IIRC the actual resistor values in CTH BOM did not exactly equal the calculated/ideal and had to be tweaked in-circuit.
Think it best to stick to a single resistor, use it & (carefully!) measure the heater voltage w/tube and see how close it is. Found this quote "The heater voltage of individual valves must be within ±7% of the rated value" here:
http://www.vintage-radio.net/forum/showthread.php?s=8d6f8e40d2b17ab465c7eafdd2573736&t=59412
Maybe order a couple/few resistors around the target values to be safe.
Noval Pin | SubMini pin | Desc |
---|---|---|
1 | 5 | A2 |
2 | 3 | G2 |
3 | 2 | K2 |
4 | 4 | H |
5 | 8 | H |
6 | 1 | A1 |
7 | 7 | G1 |
8 | 6 | K1 |