Second hand Bottlehead Crack seems too quite

[Raymond P.]

Here's how loud my Crack drives a pair of HD600's. (See picture.) I figured trying to quantify the loudness would be more useful than subjective descriptions.


The Crack's volume knob is turned to max. Source is an iPhone 5S, with volume set to the max and EQ, Volume Limit, and Sound Check all turned off in Settings. The iPhone is playing downloaded pink noise from [size=78%]https://www.audiocheck.net/testtones_pinknoise.php[/size]


(My Crack has half the Speedball upgrade - it has only the larger board on the output tube installed.)


Edit: My driver and output tubes are CV4003 and 6N13S.


-Raymond

[Tom-s]

No way Jose.  Plate voltage will rise significantly on the driver stage and leave no operating voltage for the 6080 (and potentially a ton of dissipation on the big C4S).

I still believe the OP doesn't need to mod the amp.  The next step may be doing a voltage check to see that everything is operating properly.  A dead tube would make the amp much quieter than it should be.

Thank you for correcting me. 

[Paul Birkeland]

Those brown dale resistors aren't stock parts.  I can't make out the value on them, but if the wrong value was used, that could make your Crack have very low gain.  This would also show up pretty clearly in the voltage checks.

The Speedball will nudge up the gain of the circuit a little bit also.

[RavenTooth]

I will try to replicate Raymond's setup and see if I'm in the same ballpark.

I'll also check out those brown dale resistors, and maybe take a better picture of them. Great catch Caucasian Blackplate!

[fullheadofnothing]

Resistance and voltage checks are more useful for determining if a circuit is functioning correctly than trying to measure the output with a microphone, especially when the amp in question shows numerous deviations from stock parts.

[Raymond P.]

I will try to replicate Raymond's setup and see if I'm in the same ballpark.

I'll also check out those brown dale resistors, and maybe take a better picture of them. Great catch Caucasian Blackplate!

Oh, I forgot to mention something important if you want to compare to my sound level measurement. I did add the volume pot padding described in #3 of the Crack FAQ - https://bottlehead.com/smf/index.php?topic=4295.0 My measurement would be higher without this mod.

-Raymond

[Doc B.]

How consistent is that meter reading? Does it change a lot if the headphones or meter move a little?

[RavenTooth]

Raymond, it was a little awkward because my only sound level meter is on my iPhone, so I had to use it as the source of the pink noise and also to measure the dDs, but I got a pretty consistent reading of about 87 dBs. Moving the phone around a little doesn’t really change the volume registered by a ton. So now I’m feeling more like my Crack is probably fine, or at least close.

 The brown resisters say 1451, RN70E, 2202F on them.

[Paul Birkeland]

Do you happen to have a meter so you can take some voltage measurements?

[Raymond P.]

How consistent is that meter reading? Does it change a lot if the headphones or meter move a little?

So I took measurements again, this time varying the mic's position, but always keeping it within the space between the earcups. I then tried with the headphones and meter off the table surface. The variation I get is around +/- 1db.

-Raymond

[Doc B.]

Good work. That helps to understand how well the measurement can be reproduced. Headphones are tricky to measure.

[RavenTooth]

Do you happen to have a meter so you can take some voltage measurements?

Yes I have a multimeter, altho I am no expert in using it as I didn’t build this kit. But I’m sure I can figure it out. I’ve used it to measure resistance on guitar pickups.

[Paul Birkeland]

I would start by measuring the DC voltages, which can be found in the manual.

[Deluk]

Having bought his Crack third hand RT may not have a manual. A flit through some previous problem posts should bring up some lists which give the correct voltages. Perhaps someone can point one out. Likely that a manual to fit the Speedball will also be needed at some point, maybe for the MK I 3 board version.

[RavenTooth]

FYI, I do have the manuals.

How do I fogure out the value of these non-standard Brown Dale resistors? I listed out the numbers printed on them in a previous post.

Or I guess just measure the voltage and see if it’s the same as what the manual states.