Limits of Speedball

[SchizoidMan]

So I've been looking at different tube rolling possibilities for the Crack with Speedball and it seems to accept a rather large variety of tubes. Then I found an interesting topic on the Forum regarding tube rolling with the 6528A tube.

Yes I understand that the Crack Transformer cannot handle 5 Amps of Heater Current without catastrophic results, but there was part of the discussion that caught my attention, being Speedball and Tube Trans-conductance.

I understand that the 5998 Tube for example, works well with the Crack and Speedball, but has quite a high trans-conductance relative to the 6080, and yet Speedball seems to handle that well.

So I have a few questions regarding a 6528 tube, first of which is, hypothetically speaking, if the Crack Transformer were able to handle such a High Heater Current, could a 6528 tube then function properly with Speedball, or would Speedball heat up too much and thus one would have to revert to the use Cathode Resistors, albeit with a larger power Dissipation?

Also I understand there can also be Oscillation due to this tube and have seen recommended the use of Grid Resistors in order to prevent said oscillation. I'd Imagine the Gird Resistors should go onto the 6528 Grid Pins, but I would like clarification on this, a good resistance(200 ohm?) as well as a good power dissipation rating (5 Watts or is that overkill for a Grid Resistor?)

Back to the Speedball Topic However, If in theory the Speedball board WERE to overheat due to the 6528, what would be a solution for this? Increasing Heat Dissipation rating for the 47Kohm Resistors and 31R6F resistors or Changing the Transistors to handle the Heat? TIP50 seems to handle 40W and the Heat Sinks on the board are really dang big, but maybe it's something to do with the DC Current Gain of the transistor, or is that not necessarily important in this case? Or is the whole Trans-conductance issue more of Speedball not meeting the optimal requirements for the 6528?

Also due to a 5 Amp Heater, would a Metal Tube Socket be Recommended or is Ceramic Fine? Also would 18 AWG Wire Be OK for the Heater Or would 16 AWG be better, or does it not matter too much?

Either way, Thank you for your time as well as for any responses.                                                                                                                                                               

[Paul Birkeland]

I understand that the 5998 Tube for example, works well with the Crack and Speedball, but has quite a high trans-conductance relative to the 6080, and yet Speedball seems to handle that well.

The Speedball uses an active load to provide fixed DC operating current and a high AC impedance.  Transconductance isn't a limiting factor of the Speedball's ability to work appropriately.

So I have a few questions regarding a 6528 tube, first of which is, hypothetically speaking, if the Crack Transformer were able to handle such a High Heater Current, could a 6528 tube then function properly with Speedball, or would Speedball heat up too much and thus one would have to revert to the use Cathode Resistors, albeit with a larger power Dissipation?

The Speedball will set operating current at 30mA.  The 12AU7 will apply about 80V of grid bias.  The 6528 would bias up with slightly lower cathode voltage than I would expect from a 6080, therefore the Speedball dissipation will go down, not up.

Also I understand there can also be Oscillation due to this tube and have seen recommended the use of Grid Resistors in order to prevent said oscillation. I'd Imagine the Gird Resistors should go onto the 6528 Grid Pins, but I would like clarification on this, a good resistance(200 ohm?) as well as a good power dissipation rating (5 Watts or is that overkill for a Grid Resistor?)

Sure, 200-1000 ohms is fine.  I would use 1/2W carbon composition.  5W resistors will be wirewound, and you don't want inductive grid stoppers, as they may induce more oscillations than you'd have without them.

Also due to a 5 Amp Heater, would a Metal Tube Socket be Recommended or is Ceramic Fine? Also would 18 AWG Wire Be OK for the Heater Or would 16 AWG be better, or does it not matter too much?                                                                                                                                                             

The tube sockets we provide do have metal contacts.  You'd have to test the voltage drop at the tube pins to see if the contacts are an issue running 5A of current.

You can also just buy a Crack-a-two-a, add a pair of grid stoppers, then plug the 6528 tube in.  I would estimate that the output impedance of the amp would drop to about 85 ohms.

[SchizoidMan]

Wow that was a quick reply, so I guess the biggest issue here is the heater current. I mean I'm not going to replace the Transformer on My Crack though, mostly Hypothetical and I'm Happy with my Crack as is with the RCA 5692 (with 12AU7 Adapter) and Bendix 6080, just curious after reading a post on the subject. Besides Just Put a Hammond Choke in and Cree Diodes and a 220uf Film Cap in mine and I'm honestly Super happy with it, also waiting on some Mundorf MKP Caps and a Mini-DACT potentiometer, all in all pretty satisfied with the sound. Might get the Two eventually when I'm done loving the Crack. Thanks for such a quick Reply though.

[Tom-s]

You can also just buy a Crack-a-two-a, add a pair of grid stoppers, then plug the 6528 tube in.  I would estimate that the output impedance of the amp would drop to about 85 ohms.

What's the 6.3V current limit on the C2A?

[Paul Birkeland]

What's the 6.3V current limit on the C2A?

The power transformer is much, much larger than what's in the stock Crack.  It has two 3.15V windings that we put in series to make a 6.3V winding (so think 6.3VCT) and another 6.3V winding.

With a 5A tube in the octal socket and a 12AU7 in the 9 pin socket up front, that particular winding will be pretty heavily loaded, but the rest of the amp is not so demanding of the other windings, so this is not all that concerning. 

The chassis ampacity rating for 18 AWG wire is 16A, so I wouldn't be worried about voltage drop in that short run.