Tone shift on #124 build after 30-40 mins of playing

[BobW]

I played an outdoor venue for a concerts in the park thing last Monday evening and noticed after 30 - 40 mins of playing, a slow loss of top end brilliance and sparkle when in the OD channel, 100W mode. A new PI tube fixed a similiar issue last time but the drop out was immediate and was not a slow loss.

I plan to try:
1. swapping out pre and PI tubes
2. It could be a heat related issue, in particular, the 2 paralleled 6.8K 5W resistors (choke resistors).
3. Measuring the B+ voltages, Plates and screens every 10 mins. looking for a delta measurements.
4. Using a sine wave input, dummy load (4 Ohm), in 100W OD mode.
5. makeshift a cover to let the heat buildup.
6. re-inspect / resolder suspect joints.

The 2 thiele cabs used that evening, 1 EVM 12S, and 1 EVM 12L were tested and are not suspect. I forgot to mention this is a #124 build with about 30 hours total time. The present tube complement consists of JJs for the preamp and Winged Cs for the outputs. Other than this issue, the amp sounds wonderful.
My usual settings are:
Vol 2,3
Bright, Mid, Rock On
Treble 5,6
Mid 5,6
Bass 5,6
Drive 4
Level 6
MVol 7
Presence 6

Any comments welcomed

[Structo]

Did any smoke get released?

Kind of sounds like power tubes but I suppose anything is possible at this point.
Was the clean channel affected?

Keep us posted.

Did it kill your gig when it happened?

[ChrisM]

Bias of power tubes shifting? Are they new, maybe they still havent been broken in?

[wokkel]

Is such change possible when the AC input voltage is changed?
After all it is an outdoor location.

Harald

[Normster]

Not sure how much "shift" you're talking about, but every Dumble style amp I've ever played gets sweeter after about 30 minutes. (Richer tone, easier controlled feedback, etc.)

[BobW]

No smoke was released, and didn't play the clean channel that evening. The 6L6s have about 30 hours total on them. biased at 30mA. (best setting for my tone)

The tone shift didn't kill the gig, but it sounded as if the treble was turned to 0. It lost most of the top end.

The stage power was off the city grid, but power a power dropout was possible.

Normster, for some reason, mine has always sounded best the during the first and second sets. A slight shift in tone has always been there, just not as big of a shift till now.

I forgot to mention, after initial soundcheck, the amp was left in standby mode for about 30 mins. Other than a possible heating issue, this may not be a factor.

Am busy w/ work, but will jump on it this weekend. Thanks guys for the suggestions.

[dogears]

Hi Bob,

A few observations.

1) Outdoor venue voltages can vary significantly. Did you measure the AC? My guess is you were under 114V. I use a Furman AR1215 regulator in my rack to combat this.

2) I can't imagine running the bright and mid switches on! I never have gotten an acceptable tone this way. I'd suggest relaying the bright switch to only effect the cleans. Maybe a 120pf.

3) Try upping the master volume bright cap. A minimum of 33pf should be there for live use and will be perfect for a master at 7. This will restore clarity. Do you use the loop? If so, raise bright cap to at least 47pf. Try a 6 foot good cable in the in/out jacks if you do not use a 'lator.

4) Input volume needs to be past noon. I am pretty certain #124 had a 30% taper pot there. If you have a 15% taper, run the input at least at 1 oclock. Minimum. This will negate the need for midboost.

5) Set OD trimmer no more than 40-45%. Try lower, like 30%. Then raise input gain to compensate.

6) Try something like 2.2K or 2.7K for the first post PI dropping resistor. Try to get 200-205V on V2.


Let us know what happens!

I played an outdoor venue for a concerts in the park thing last Monday evening and noticed after 30 - 40 mins of playing, a slow loss of top end brilliance and sparkle when in the OD channel, 100W mode. A new PI tube fixed a similiar issue last time but the drop out was immediate and was not a slow loss.

I plan to try:
1. swapping out pre and PI tubes
2. It could be a heat related issue, in particular, the 2 paralleled 6.8K 5W resistors (choke resistors).
3. Measuring the B+ voltages, Plates and screens every 10 mins. looking for a delta measurements.
4. Using a sine wave input, dummy load (4 Ohm), in 100W OD mode.
5. makeshift a cover to let the heat buildup.
6. re-inspect / resolder suspect joints.

The 2 thiele cabs used that evening, 1 EVM 12S, and 1 EVM 12L were tested and are not suspect. I forgot to mention this is a #124 build with about 30 hours total time. The present tube complement consists of JJs for the preamp and Winged Cs for the outputs. Other than this issue, the amp sounds wonderful.
My usual settings are:
Vol 2,3
Bright, Mid, Rock On
Treble 5,6
Mid 5,6
Bass 5,6
Drive 4
Level 6
MVol 7
Presence 6

Any comments welcomed

[tubedogsmith]

For something as drastic as your describing I doubt it's a circuit problem, I'd guess low AC voltage or a bad tube. Turn it on when you get home from work and just leave it on for few hours and see what it sounds like after. It should get plenty hot enough so if it's a problem with the amp it'll show up.

[BobW]

Did you measure the AC? My guess is you were under 114V. I use a Furman AR1215 regulator in my rack to combat this.

The regulator is now on my list for new gear. I wasn't able to measure it, but will measure stage voltages during next weeks performances. I know the crew, they won't mind.


4) Input volume needs to be past noon. I am pretty certain #124 had a 30% taper pot there. If you have a 15% taper, run the input at least at 1 oclock. Minimum. This will negate the need for midboost.

5) Set OD trimmer no more than 40-45%. Try lower, like 30%. Then raise input gain to compensate.

I must have misread posts in the past, and assumed the #124 blooms better with the Vol at 2,3...then adjust the Drive, Level, and MVol accordingly? Setting the Vol to 12 or 1 o clock as you suggest indicates my OD trimmer is set way too high. Although I do remember measuring it. that also explains why the clean channel is almost unuseable at 2,3.

Will try out your other excellent suggestions, and report back. Thank you sir!

[BobW]

This weekend I checked the following (not in any particular order:
OD trim is set to approx 25%
V1a,b = 205, 208
V2a,b = 204,208
Plates = 442
V3a,b 275,278
tried 33pF on the MVol, not much difference vs. 15pF

The tone is somewhat restored FWIW, but could be improved.
Using stock #124 Orange drop cap values
Pots measured:
Vol 1.04 M
Treble 247K
Mid 269K
Bass 497K
Drive 255K
Level 101.2K
MVol 1.02 M
Presence 4.97K
mostly used in 100W mode


Using Dogears suggested OD settings did help a bit but noticed the clean channel is nothing spectacular. It has plenty of Bass but could use a little more Mid and Treble. Bright and Mid were off, in Rock mode, but not much note bloom when the mid boost is off. The wiring was double checked, and found no issues. Using a good 6' cable in the loop (adding a small amount of capacitance) made no difference, since I need more mids and treble. Although both the EVM12L and S loaded Thiele cabs sound great using other amps, (Vibrolux, Deluxe, Super Reverb) I will tryout a different cab 2-12 bassman. Has anyone tried poly caps in the tone stack, and was successful in improving note bloom and touch response?
I will have an opportunity to measure the stage voltages tonight.

[UltraHookedOnPhonix]

I use a Furman AR1215 regulator in my rack to combat this.

Quick offshoot here...

Here's what Furman has to say about the AR1215:

“The AR1215 is intended for live sound ONLY. We use a 10-Pound autoformer to convert the voltage and this model can cause mechanical hum from the unit. If you require quiet operation we suggest using the AR-15 Series II”.

The AR-15 Series II is a discontinued model but I suppose the new Furman P-1800 AR is it’s new replacement. There’s also a unit that ETA makes, model PD10VRS that I’ve seen Scott use in his rack. How do the AR1215 and PD10VRS compare Scott?

Here’s the datasheets for the PD10VRS and the new Furman P-1800 AR:

[ChrisM]

Just a note, but your presence should be 2K.

You can make the pot 2K by putting a 3K3 across the outer lugs.

[BobW]

Just a note, but your presence should be 2K.

You can make the pot 2K by putting a 3K3 across the outer lugs.

Chris good catch, Can't believe I missed that, will go back and re-measure. If it's wrong, then the 3.3k will help.

Ultra, thanks for the Furman data sheets.

[dogears]

Hi Bob,

What tubes are you running in V1 and V2? Try some high gain JJ ECC83S. Also, use a balanced in V3 and set the trimmer to a baseline of about 15K between the plates to start tweaking from.

I would put the 100K pot in for the OD drive. Have you compared between it and the unique 250K in 124? I have NEVER preferred 250K in any build, except HRM.

Let us know!

[dogears]

I have both the AR1215 and the ETA PDVR10S. I have the AR1215 in my rack since it is about 10 pounds lighter!! The ETA is a higher quality unit though. It is more solid, but deeper and heavier. It has a slightly tighter range I believe as well. Just a beast, weight wise.

I have never had noise issues with my AR1215.

Apparantly the older units had 3V steps on the tranny. The 5V steps on the new one do little to tightly control the AC to the degree I would want. I have seen 116 to 121.5 or so. I would want 118-121. Nice 3V range....

I use a Furman AR1215 regulator in my rack to combat this.

Quick offshoot here...

Here's what Furman has to say about the AR1215:

“The AR1215 is intended for live sound ONLY. We use a 10-Pound autoformer to convert the voltage and this model can cause mechanical hum from the unit. If you require quiet operation we suggest using the AR-15 Series II”.

The AR-15 Series II is a discontinued model but I suppose the new Furman P-1800 AR is it’s new replacement. There’s also a unit that ETA makes, model PD10VRS that I’ve seen Scott use in his rack. How do the AR1215 and PD10VRS compare Scott?

Here’s the datasheets for the PD10VRS and the new Furman P-1800 AR: