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DT50 Tube Bias Variation


leperclown
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Hi all,

 

A year ago, I had my DT50 1x12 repaired, fully retubed and biased by a Line 6 authorised tech here in Melbourne Australia.

 

I hadn't used the unit much over the last year, but got it out to try the amp and also see how it would perform using my Boss GT1000 direct into the return/power amp in socket.

 

Before doing this, I decided to re-bias the amp. (I have been doing this for the past 20 years with a wide variety of tube amps. I have the necessary gear and the confidence to perform this procedure). However, usual warnings about the potential FATAL consequences of servicing tube amps - Don't open up and work on a tube amp unless you are ABSOLUTELY sure of what you are doing.

 

While re-biasing the DT50, I noticed there was a significant difference between:

  • The bias reading from the internal test points (3 prongs); and
  • The bias reading directly from the power tubes (using 'plug-in, in-line probes)

 

For example, for my final settings (recommended: 35mv per tube) I got:

  • Internal test points - V1: 42      V2: 44 
  • In-line-probes -         V1: 34      V2: 36

 

Based on past experience, I have high confidence in the accuracy of the in-line probe readings.

 

It would appear that, if you rely on the internal test point readings, the DT50 might be biased too 'cold' (by 8mv). This could result in the power amp sounding lifeless, with reduced dynamics. The implication is that, using the internal bias points, if you want to actually bias to 35mv a tube, each tube would need to be set to 43 mv a tube, which is a little scary.

 

My Questions:

  • Is there a basic flaw with my analysis?
  • Has anyone else noticed this variation?
  • Is there an explanation for the significant variation between the test point and in-line tube bias readings?
  • Which reading would you trust - internal test points or in-line bias probe?

 

BTW, the DT50 sounds amazing on its own (particularly stock voicings I & IV) and it is an excellent power amp for the GT1000 (or other modellers I would imagine). It's a pity it is so big (same width but taller than my Marshall JVM 410 100w 2x12 combo) and so bloody heavy!

 

Cheers!

 

Paul

 

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Using matched pair EH tube mediums at 35mV bias is the way to go.

Do not go outside the bias range, that would be an unsupported alteration.

 

Authorised service center has access to the bias and burn-in procedure document for that product.

If for some reason they don't, they can get it from me through tech channels.

 

Also check my post here on the subject of tube replacement for DT series amps.

 

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