Hot Rod Deluxe low power amp output

[Tone Meister]

Amp is an early USA Hot Rod Deluxe. Amp works and sound ok but it's evident that output power is low when playing the amp. Owner said output is ok when first turned on but fades after heating up. Amp is lacking the girth and volume of a stock amp.

Using a Fluke 87 multimeter, I measured 12 Vrms (~18 Wrms) output under schematic test conditions. Schematic OUTPUT TEST indicates output should be 16.3 Vrms with a 263 mV signal introduced at J4 and connected to an 8-ohm resistive load.

AC line voltage = 119 V
B+ at C31 = 423 vdc (431 on schematic)
Bias = 68 mV

Pulled the power tubes and B+ went up to 453 vdc. Tried a second pair of known good power tubes with same result. Amp sounds the same through an extension cabinet with 2x12's.

The amp has clearly seen a great deal of work in its lifetime. All new filter caps, a poorly fitted power transformer, and other misc recognizable work.

[g1]

Check that both PI plates have their correct DC voltage. (V3 pins 1&6)

[52 Bill]

The usual suspects are the power amp in jack, the extension speaker jack. Be certain that the normally closed contacts are clean and passing signal.

If you either measure the preamp out or listen to it through another amp, does it seen strong enough? How about if you send a signal into the power amp in jack?

If the power transformer has been replaced, what has it been replaced with?

[Tone Meister]

V3
pin 1 = 242 vdc
pin 6 = 250 vdc

Replacement PT is a ClassicTone 40-18015

Just cleaned the power amp in and extension speaker jacks and checked the ext speaker jack visually for oxidation or physical damage. Will test the preamp and power amp separately first thing in the morning.

[Tone Meister]

Tested voltages through test point 30 as per test conditions. Definitely something amiss in the preamp.

Measured readings are first and schematic values in parentheses.

TP1 = 4.1 mV (4.0 mV)______________TP16 = 13.6 mV (39.0 VAC)
TP2 = 145 mV (100 mV)_____________TP17 = 257.8 mV (1.28 VAC)
TP3 = 1.6 VDC (1.72 VDC)___________TP18 = 228.4 mV (1.14 VAC)
TP4 = 12.3 mV (13.0 mV)____________TP19 = 197.1 mV (870 mV)
TP5 = 335 mV (252 mV)_____________TP20 = 11.4 V (11.6 V)
TP6 = 1.5 VDC (1.72 VDC)___________TP21 = 4.5 mV (43.0 mV)
TP7 = 47.5 mV (33 mV)_____________TP22 = 12.6 mV (3.43 V)
TP8 = 513 mV (850 mV)____________TP23 = 143.8 mV (744 mV)
TP9 = 3.5 VDC (1.72 VDC)__________TP24 = 2.6 V (11.5 V)
TP10 = 630 mV (550 mV)___________TP25 = 2.2 V (11.5V)
TP11 = 8.3 V (13.3 V)______________TP26 = 35 VDC (37.2 VDC)
TP12 = 1.5 VDC (1.72 VDC)_________TP27 = 13.6 V (66.5 V)
TP13 = 243.8 mV (1.2 V)___________TP28 = 13.7 V (66.5 V)
TP14 = 13.8 mV (66 mV)___________TP29 = 1.1 V (5.34 V) extension speaker jack
TP15 = 13.8 mV (66 mV)___________TP30 = 68 mV (60 mV) bias

[52 Bill]

Tested voltages through test point 30 as per test conditions. Definitely something amiss in the preamp.

Are your test voltages with the amp in the clean mode?

The first low reading comes after the relay K2A. Read the resistance across the contacts and see if it has a problem.

[Tone Meister]

Test point conditions are as follows. Drive (yellow) mode.

Reverb control fully CCW
All other controls at 50% rotation
Bright switch out
Drive (Yellow) Mode

[52 Bill]

Well then the problem seems to be in the power amp. The signal is low at the plates of the driver tube.

How do the dc voltage in that stage read compared to the schematic? Check cap C30, I've seen that one go bad before.

Edit: I'm sorry, I'm distracted here. There are low readings in the preamp as well that need to be resolved.

[Tone Meister]

The first low reading comes after the relay K2A. Read the resistance across the contacts and see if it has a problem.

Yes, the NC closed contact (R18 to R28) has continuity

[Tone Meister]

Well then the problem seems to be in the power amp. The signal is low at the plates of the driver tube.

How do the dc voltage in that stage read compared to the schematic? Check cap C30, I've seen that one go bad before.

The power amp was my initial suspicion, yet many of the preamp measurements are out of whack.

As per the OUTPUT TEST on the schematic, I injected a 263 mV sine signal at J4 POWER AMP IN and tested output. The power amp is only producing about 18 Wrms (12Vrms @ 8 ohms resistive) as opposed to an expected 16.3Vrms

[52 Bill]

You've got a typo in that last post? What is the real output voltage?

One of the problems with testing this way, is that the figures are all variable based upon control settings, etc.

Edit:Wait I just re-read the earlier posts. 12 volts versus 16 volts isn't all that far off, especially with a lower than normal B+ voltage supply.

Have you tried reducing the bias to see how much the B+ changes?

[Tone Meister]

Bill,
Just found one culprit in the preamp. The U3 4560 op amp has DC on all the outputs so I need to socket that and replace the chip before going any further.

The output difference in the power amp translates to 18 Wrms actual versus 33 Wrms expected output. I can feel and hear a difference in this amp as opposed to a healthy stock HRD.

Will reduce bias and recheck the B+ before going any further.

[52 Bill]

I'd hold off on that, U3 is part of the channel switching circuit, so it will have dc on all of the pins. In general with these amps, the IC's are completely out of the audio path unless the signal is being sent through the FX loop.

Please post the dc readings of the different power supply nodes B+, Z, etc.

[Tone Meister]

Line 120 VAC
bias 60 mV
B+ = 431 V
Z = 429 V
Y = 389 V
X = 333 V

[Jazz P Bass]

What is the Vac signal amplitude at the 6L6 grid pins at full output?