8/10/2019 DT25 Power Amp Parameters

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DT25 Power Amp Configuration Dependent

Output Levels(& hidden Power Amp Controls)

Using a simplepower amp only test tone

(preamp in path A drives power amp settings, but path A

muted in mixer, path B is used to pass unprocessed input signal, mixed to center and routed to the power

amp) one can observe the volume and any sound impact of the power amp voicing/class/tube configura-

tion (and other HD parameters) may have on the DT25 power amp section with an SPL meter recording

RMS and peak levels.

There are 6 voicing-class-pentode combinations used by the available HD preamp models, but the SPL

levels indicate that there are 8 (groups of) preamps producing different power amp output levels and 4

more can be selected when the voicing-class configurations are extended to use combinations not selected

by the existing HD preamp models.

Below are the identified (groups of) preamps listed from quietest to loudest on the basis of A weightedSPL RMS measurements using a medium fast repeated single guitar tone (JTV-69 Spank 4) as input:

GroupType

VoicingClass

AdjustLevel

(List of) Preamps selecting the this voicing/class configuration

1 I, A/B 0.0 dB Blackface Dbl & Lux, Flip Top, Hiway 100, Solo-100, Line 6 Epic

2 I, A -5.0 dB Group 1 with class changed to A (not selected by any preamp)

3

I, A/B

-6.0 dB

PhD Motorway, Line 6 Doom

4 III, A/B -6.0 dB Class A-15 with class changed to A/B (not selected by any preamp)

5

II, A

-7.0 dB

Super O

6 II, A/B -9.5 dB Tweed B-Man, Plexi Lead 100, Brit J-45, Brit P-75, Brit J-800

7 IV, A/B -10.5 dB Angel F-Ball

8

I, A

-11.5 dB

Group 3 with class changed to A (not selected by any preamp)

9 III, A -12.0 dB Class A-15

10 III, A/B -13.0 dB Divide 9/15, Bomber Uber Treadplate, Line 6 Electrik

11

IV, A

-15.0 dB

Group 6 with class changed to A (not selected by any preamp)

12

III, A

-19.0 dB

Gibtone 185, Class A-30TB

The table does not cover the power amp configurations using a triode setting which will add up to 12

more variations for a grand total of up to possibly 24 HD500 selectable power amp configurations, which

is 50% more than the 16 one would think of when looking at just voicing I-IV, class A and A/B, and pen-

tode and triode combinations.

8/10/2019 DT25 Power Amp Parameters

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But there is more power amp stage control through the PRES dial on the HD500. While checking amp

settings I noticed that the PRES dial does have some unexpected impact on the power amp behavior for

all voicing types except III. The magnitude of PRES settings on output level is amp configuration depen-

dent and observable starting around PRES values > 50% and gets stronger in non-linear fashion with in-

creasing PRES values (the effect does not exist for type III voicing configurations). My measurements

confirm that DRIVE, BASS, MID, TREBLE, CH VOL, ER, microphone, cabinet and cabinet parametersettings have no impact on the power amp stage output level or tone as far as I can tell.

To summarize, the HD500 and DT25 (and probably the DT50) seem to support at least 2 undocumented

power amp control features that are (and thus can be ) indirectly selected with the HD500 by choosing

select amp models and PRES setting above 50% (for all but type III preamp voicing types).

My averaged peak signal and RMS amplification level measurement results for all preamp model groups

and PRES settings from quietest to loudest at 50% and 100% PRES settings are as follows:

Group

Type

Voicing

Class

50% PRES 100% PRES HD preamp model selection

(see full preamp list per group in table above)

Peak

RMS

Peak

RMS

1 I, A/B -20.0 -34.0 -15.0 -31.0 Blackface and other I, A/B preamps

2 I, A -15.0 -29.0 -9.0 -26.0 Group 1 preamps changed to I, A

3 I, A/B -14.0 -28.0 -9.0 -25.0 PHD Motorway & L6 Doom

4 III, A/B -13.0 -28.0 no change Class A-15A changed to III, A/B

5 II, A -14.0 -27.0 -8.5 -24.0 Super O

6 II, A/B -12.0 -24.5 -7.0 -22.5 Brit and other II, A/B preamps

7

IV, A/B

-10.0 -23.0 -7.5 -22.5

Angel F-Ball

8

I, A

-9.0

-22.5

-4.0

-20.0

PhD Motorway & L6 Doom changed to I, A

9 III, A -7.0 -22.0 no change Class A-15A

10 III, A/B -5.5 -21.0 no change All III, A/B preamps

11 IV, A -6.0 -19.0 -3.0 -18.0 Angel F-Ball changed to IV, A

12 III, A 0.0 -15.0 no change Class A-30TB & Gibtone 185

Notes: SPL meter not calibrated (should not matter much because only relative difference maters)

All measurements taken with a loop of medium fast repeating single notes (JTV-69 Spank 4)

Repeat measurements varied < 0.5 dB for same loop material

Typically measured 3+ times, averaged, and rounded to next 1/2 dB

Used different loops over several months of checking (results are a mix of measurements)

The order of lowest to highest peak SPL signal levels does not quite match the order of quietest to loudest

RMS signal levels, and the order is changing depending on the selected PRES values, i.e., the table above

is just a starting point for adjustments to correct for different amplification levels.