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Cavalli Compact Tube Hybrid (CTH) Tube & Tweak thread. - Page 46

post #676 of 685

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.  frown.gif

post #677 of 685

Sorry for kicking a somewhat dead thread but I had a question and thought it might be better to ask here than start a new one.

 

I have a small guitar practice amp that I'm very happy with except that it doesn't have a headphone jack for late night practicing.  It does however have a preamp out.  Can I run the preamp out of the guitar amp into the input of the CTH?  It would require the proper cable, e.g., 1/4" to RCA or 1/8" (I have both inputs on my amp). 

post #678 of 685
Thread Starter 

If you start with the volume all the way down when you try I don't see that harm could happen. 

When the volume is all the way down the CTH's input is swept to ground (it doesn't "see" any input).

And the onboard e12 should trip if anything goes wrong. 

IOW I'd give it a whirl, granted its lower risk for me because I'm a builder:)

 

Edits:

* If its a mono amp then the preamp out is likely mono too.  In which case your adapter(s) would need to link the preamp's single channel "+" to both the CTH's L & R "+"s (to hear sound in both earpieces).

* With Joe Audio now offering CTH PCBs & kits ( http://joeaudiophile.com/shop/ ) the thread could see some posts. Tho most topics are already covered.


Edited by cfcubed - 1/11/13 at 4:02am
post #679 of 685
Quote:
Originally Posted by cfcubed View Post

IOW I'd give it a whirl, granted its lower risk for me because I'm a builder:)

 

And you're the builder of my amp also.  If the amp fails I'm going to track you down and try to get you to fix it.  :)

post #680 of 685

I've been using the CTH with an Amperex Bugle Boy ECC82 to drive Audeze LCD-2s. The bugle boy has given me the most extended treble, from among Mullard, Telefunken and older Amperex tubes. Does anyone know of another tube that's even more extended in the treble, to balance out the LCd-2?

 

Also, has anyone heard both the CTH and the Schiit Lyr and compared the sound quality?
 

post #681 of 685
Quote:
Originally Posted by cfcubed View Post

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.
post #682 of 685
Thread Starter 
Quote:
Originally Posted by Tribbs View Post
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. 


Edited by cfcubed - 2/2/15 at 1:12pm
post #683 of 685
Quote:
Originally Posted by cfcubed View Post


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. 

Thanks for the prompt reply CF3. I've been working on this amp in very intermittent spurts LOL.

Yes, I've ordered a few Rs. Your measuring tip is very useful - thanks

BTW, I have printed out a full one page schematic I saved many years ago that indicates 7.15K for 6.3V instead of the above 7.4K (7.39K) next to the heater switch.
Do you know which is correct or...?

Edit: I just noticed BOM (3/20/2011) lists 7.15K, I have Xicon's on their way!

Many thanks for this and keeping up with the thread.
Edited by Tribbs - 2/17/15 at 7:25pm
post #684 of 685
Thread Starter 

See you found it & edited your post about the CTH Rev A BoM:

http://cavalliaudio.com/diy/ctha/main.php?page=parts/excelbom

 

Looking at BoM reminded me that "single resistor" in my post means single resistor in parallel with R1H:

 

post #685 of 685
If anyone has the time to help me debug a problem I posted in the other thread it would be greatly appreciated:
http://www.head-fi.org/t/398839/a-very-compact-hybrid-amp/2205#post_11416293
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