New build sounded good, now sounds like a long string of firecrackers

[pullshocks]

I recently finished a build from the SoLow Watt design that has been built by several on the Hoffman and AmpGarage forums. It is a push pull amp. I used 6F4P tubes instead of ECL84. Schematic attached. All new parts, except the tone caps and PI coupling caps. I got the 6F4P from a member on the Wattkins forum, don’t know anything about them. Output transformer is a Hammond 125E, wired for 17K to 8 ohms.

It worked first time, and sounded great for a couple hours. Now it sounds like a long string of fire crackers going off, kind of like 4th of July in my neighborhood. In retrospect there were a few isolated pops at first. After maybe 2 hours of playing and maybe 10 off/on cycles, the popping got worse and worse. It would start 30-50 seconds after initial turn on. While the popping is going on, voltages are unstable, and one or both of the 6F4P are strongly microphonic. Tapping anywhere in the chassis or amp it is microphonic.

With a guitar plugged in, it makes sound, but sounds awful when the popping is going on.

For now I will skip over my initial attempts to trouble shoot. I now think it is an issue with the tubes. As of last night I found one of my 6F4P is shorted between grid and cathode. With my remaining 3 6F4P, the popping starts after 30-50 sec of warm up. In the dark, the 6F4P blue glow gets brighter as the pops happen. The onset of popping seems to occur as the tubes get warmed up
The popping behavior occurs even with the 12AV7 preamp tubes pulled.

I am going to order up some NOS ECL84/6DX8 tubes.

To reiterate, the amp seemed to work well for a couple hours. But I wonder if the 6F4P just have a high failure rate, or if something about the build is stressing them.

Thanks in advance

[Stevem]

The microphonics is telling you that there is a structural problem with the output tubes, and that is the source of the arcing and inturn the popping that your hearing.

I would also check your PI output coupling caps for signs of DC voltage leakage.

[Synchu]

While I built this one, I never kinda bonded with the original design and sound of these 'combo' tubes. Added another double triode and 'proper' power pentodes (cheap, abundant and great sounding 6P1P Russian tubes) and it turned out a great little amp (plus some 'options' that I added along the way ). Here's what's pretty much the final schematic of my build - https://ampgarage.com/forum/viewtopic.p ... 40#p279540

Now besides the above suggestions, I would check filaments wiring and voltages, as well as look at and use a wooden chopstick to poke around the power supply wiring.



Niki

[pullshocks]

The microphonics is telling you that there is a structural problem with the output tubes, and that is the source of the arcing and inturn the popping that your hearing.

I would also check your PI output coupling caps for signs of DC voltage leakage.

Thanks Stevem. New tubes on order.

As far as the PI output coupling caps, other than pulling them out and checking capacitance and resistance with my DMM, is there any other test I can do?

[pullshocks]

While I built this one, I never kinda bonded with the original design and sound of these 'combo' tubes. Added another double triode and 'proper' power pentodes (cheap, abundant and great sounding 6P1P Russian tubes) and it turned out a great little amp (plus some 'options' that I added along the way ). Here's what's pretty much the final schematic of my build - https://ampgarage.com/forum/viewtopic.p ... 40#p279540

Now besides the above suggestions, I would check filaments wiring and voltages, as well as look at and use a wooden chopstick to poke around the power supply wiring.



Niki

Thanks Synchu. I have verified 6.3 volts on the heaters. Chopsticking -- literally every wire, component, and the chassis itself is microphonic when the popping is going on, which pretty much constant now. The 6F4P are most sensitive of all.

I will have to read your thread about your circuit and tube changes. I do have an unused nine pin socket in the build, if it comes to that. But I want to get the amp working in its current form and play it for quite a while before making major changes.

[Phil_S]

As far as the PI output coupling caps, other than pulling them out and checking capacitance and resistance with my DMM, is there any other test I can do?

Check for DC leakage. Leave the caps in circuit. Clip your meter probes to each side of the cap. Check for DC voltage. Caps should not pass DC, only AC. If there is any significant amount of DC, the caps are leaking and need to be changed.

[pullshocks]

As far as the PI output coupling caps, other than pulling them out and checking capacitance and resistance with my DMM, is there any other test I can do?

Check for DC leakage. Leave the caps in circuit. Clip your meter probes to each side of the cap. Check for DC voltage. Caps should not pass DC, only AC. If there is any significant amount of DC, the caps are leaking and need to be changed.

Thank you.
Do you mean with the amp on and tubes in, or with the amp turned off ?
Do you mean check for DC from each lead to ground, or from one cap lead to the other?

I tried it with the amp turned off and a jumper/bleeder resistor from B+ to ground. There was no DC reading across the 2 leads the tone stack caps. There was a reading of a few mV across the 2 leads of the coupling capacitors between the PA and PI.

The tone stack caps are orange drop (don' remember which product code). The 2 coupling caps are EDIT: Mallory 150, rated 630 volts

[Phil_S]

You can't check for voltage with the power off and the tubes pulled. This test is an operating condition test. Put the tubes in. Power on. One probe on each lead of the cap, meter set to DC. If it is not an autorangeing meter set for the mV scale. I like to use clips on my meter probes because then I don't have to be concerned about slipping.

[bepone]

@pullshocks
there is no any picture of build or voltage measured, how we can help?
obviusly something is wrongly connected, amp is burning the tubes out..6F4P are russian if i understand well, so this means voltage cannot be high.

so lets start from 0, remove output tubes which i assume are burned because of some wrong connection there, start the amp and write at least cathode voltages in preamp , so we can see the situation.

for second step, will be the best to put signal in the input (sine 100mVpp), without output tubes in, and see the preamp output /eff loop jack/ with the scope if is ok.

[pullshocks]

@pullshocks
there is no any picture of build or voltage measured, how we can help?
obviusly something is wrongly connected, amp is burning the tubes out..6F4P are russian if i understand well, so this means voltage cannot be high.

so lets start from 0, remove output tubes which i assume are burned because of some wrong connection there, start the amp and write at least cathode voltages in preamp , so we can see the situation.

for second step, will be the best to put signal in the input (sine 100mVpp), without output tubes in, and see the preamp output /eff loop jack/ with the scope if is ok.

Thanks for being willing to help. Pictures attached. As far as voltages, they have been pretty eratic and inconsistent, and unfortunately I did not measure voltages before the popping problem started. I have ordered a set of NOS 6DX8/ECL84 tubes. When they get here I will do a full set of voltage measurements, and try the capacitor leak test suggested by Phil S

My newly acquired tube tester shows 2 of the 6F4P have shorts. The remaining 2 test "bad" on the tube checker. As a result, I think it is best to wait until I have some known good tubes to work with. But it appears the pentode plate voltages are in the range of 250 volts, pretty close to the B+ value in the original schematic.

I think you may be on the right track as to voltage possibly being too high for the Russian tubes. The amp circuit is known to work with 6DX8/ECL84. 6F4P is supposed to be an equivalent, but maybe it has a lower max voltage. I can't find a data sheet in English.

On the Chasing Tone web site there are some 6F4P amp builds running at a plate voltage of 229V. https://chasingtone.com/category/6f4pamps/.

I am pretty certain the issue is not in the preamp. pulling the preamp tubes did not affect the problem.

.

Socket V3 and V4.jpg

P1010719 700 pixel.jpg

P1010713 700 pixel.jpg

[bepone]

ok.. but lets start from 0..
start the amp again , without power tubes and measure the voltages..

Ub+ and all cathode voltages.. you know that all is ok in preamp when cathode voltages are in range 1-2V /without looking at the schematic at all /

when you confirm that you can go forward.. the best is the scope method but if you doesnt have there is also multimeter method, attach VAC on multimeter on the loop output signal, and play guitar.. you can se some reacting voltage, proportional to your playin in range 5-10V..
so you can verify the preamp

[bepone]

for the oputput section i need to check how is wired 6F4P..
datasheet is here
http://www.gstube.com/data/695/

[pullshocks]

With V1 and V2 (both 12AV7) only,

B+ readings were steady at 278-266-249-239-261 V. The last node is higher because it is paralleled off Node B.

V1 (wired in parallel) plates 129 V cathode 4.3 v grid .1 mV

V2 gain stage with cathode follower

pin 1 83 V Pin 2 .4 mV Pin 3 2.35 V Pin 6 238 v Pin 7 .4 mV Pin 8 90 v.

Received the Sylvania 6DX8. No more firecrackers, amp sounds about the same as it did originally. B+ with all tubes in is 255 232 210 201 227. The pentode plate, screen, and cathode voltages are pretty close to those reported by the designer 255 v plate, 230 v screen, 4 v cathode, .

Next, I'll see if anything changes with a few hours of playing time, i.e. whether the new tubes develop firecracker issues like the 6F4P did.

[bepone]

good! cathode voltage, bias is normal, 4V.
i forgot to mention that many russian tubes compared to western have different bias (they are not 100% interchangeable) and they need to correct that.. probably was needed here to change little bit cathode resistor, or they were just bad tubes