Safe to pull Phase inverter tube??? - Page 1
Forum section: electronics / projects

author: mkiijam
author_rank: Regular Contributor
timestamp: October 05, 2025, 02:13:30 am
content: Is it okay to pull a phase inverter tube on a guitar amp for powered up testing with everything else installed? I'm getting conflicting information. I'm getting some intermittent crackle and would like to isolate where it is coming from.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: October 05, 2025, 11:01:30 am
content: No.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 05, 2025, 12:45:28 pm
content: I think he means to pull the tube and then try powering it up, he doesn't mean hot swapping the tube.I would say it should be fine, a pulled tube and a burned out one are more or less the same and it would be a really poor design if a single burned out tube caused it to self destruct. For additional protection, a current limited supply can be used.

author: mkiijam
author_rank: Regular Contributor
timestamp: October 05, 2025, 01:51:05 pm
content: Yeah for sure... I've always heard and tried pulling pre amp tubes and running just fine, just can't seem to find anything on the phase inverter

author: TimFox
author_rank: Super Contributor
timestamp: October 05, 2025, 02:32:11 pm
content: Are the phase-inverter heater and other heaters wired in series?

author: trobbins
author_rank: Frequent Contributor
timestamp: October 05, 2025, 11:24:18 pm
content: I'd initially say it's not ok, as you haven't identified the amplifier or provided a schematic. Not all valve amp circuits use the same phase inverter circuit, and there is always the possibility that your amp has an uncommon circuit where the output stage bias or some other operational characteristic could be affected, or as TimFox highlighted.Perhaps worthwhile to use electrical contact spray on the socket pins and use multiple valve insertions to clean the pins/terminals for starters. Then with no input signal, physically joggle the valve in its socket and listen for intermittent crackle - perhaps with volume initially low for starters. Then do a few valve swaps with other known good valves. That may avoid the need to probe for internal voltages if you are not adequately experienced.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 06, 2025, 12:30:14 am
content: Quote from: TimFox on October 05, 2025, 02:32:11 pmI would think that rather unlikely for a guitar amp. Probably has a proper filament winding on the power xformer.For a 5-tube radio, sure.Besides, if that were the case the thing just wouldn't light up if you pulled a tube so no harm, no foul.

author: TimFox
author_rank: Super Contributor
timestamp: October 06, 2025, 03:42:19 am
content: That's why I asked.Some non-AA5-radio circuits put 150 mA heaters in series with the output tube cathodes to reduce hum.

author: mkiijam
author_rank: Regular Contributor
timestamp: October 06, 2025, 04:27:20 am
content: The noise is a random "pffffft------crackle----pf-pf-pfffffffffPPPPFFFFPOP-pppft type thing. It appears on my scope at C61 and C62 and they are out of phase with each other with makes me think that it is at least partial going throught the PI tube. It's on C60 in the presence circuit which makes sense as that is a tape from the output.It does not appear at C56.

author: gbaddeley
author_rank: Regular Contributor
timestamp: October 07, 2025, 05:02:01 am
content: Joggling valves / tubes while powered up is not recommended. If a pin connection is lost momentarily it can cause a very high voltage spike which can damage down stream valves, passives, transformers and speakers. I’ve seen it happen. The perpetrator thought is was an innocent manoeuvre. They paid for the repairs.

author: mkiijam
author_rank: Regular Contributor
timestamp: October 15, 2025, 10:08:07 pm
content: I'm not planning on pulling the tubes why it is powered up. I'm talking about the standard practice of running an amp without tubes in it for testing. I know that it is okay to do this with preamp tubes out. I know this to be okay with power tubes out. I know this to be okay with all tubes out. But what about everything in except the phase inverter tube?

author: floobydust
author_rank: Super Contributor
timestamp: October 15, 2025, 10:24:25 pm
content: It's just silly to run things pulling tubes out. A bad habit, do not adopt.Generally speaking, with the phase-inverter pulled, nothing driving the output tubes, it will be noisy with hum because those grids are floating and picking up whatever is nearby. Trouble can happen depending on the feedback loop, the amp can oscillate and wreck itself as usually the phase-inverter is in the feedback loop which when open, is not stable.If you want to find a noisy tube/bad socket connection, you wiggle them to flush that out. Or swap the tube.Another approach is take a plastic pen and start tapping things. You could have a dirty relay contact, pot, tube etc. and violence does wonders to find the misbehaving part.

author: mkiijam
author_rank: Regular Contributor
timestamp: October 15, 2025, 10:56:42 pm
content: Pretty sure it's not the tube or tube socket at this point. New tube and have exercised the socket multiple times. The noise is blatant on C62 on both sides. It is lower after R164, meaning it's not coming from the BIAS relay. The plate resistors have been changed and I'm beginning to suspect that relay that is in the cap switching circuit off C62, but Mesa amps are notoriously time consuming to remove PCB's. I suppose I could run it with all the tubes out....?

author: floobydust
author_rank: Super Contributor
timestamp: October 15, 2025, 11:17:23 pm
content: Pull the PI tube... the noise is either ahead of the stage, in it, or after it. Does it narrow anything down?What does it do with muting off/on?I'd measure the DCV on the phase-inverter tube pins and see if the noise can be seen as readings moving around.A bad coupling cap and they will leak and upset grid bias. Also measure DCV on the output tubes' control grid to see if the bias there is noisy.You could ground one side of R209 22k to short out signal there. I don't know where the JFET gate goes, if it has gone noisy or whatever feeds it is noisy.With the amp off, you could measure resistance across some coupling caps and see if they are leaky, although Mr. Carlson has a special jig for that. You have to test them "at voltage" I think.Mesa Boogie have big differences "surprises" on their schematics verses the actual build. Careful with that.

author: mkiijam
author_rank: Regular Contributor
timestamp: October 15, 2025, 11:53:50 pm
content: R107, R108, C61, C62,C100,C110,C115 have been replaced.If I engage the MUTE then the noise is only on C62 and of course downstream, but since the input to V7B is a function of the FB circuit, I would assume that it is NOT on the input to V7A. If it was, then MUTE would kill the signal into V7B. Double check, but I think that is correct logic.The DC voltages look pretty stable on all the pins.

author: floobydust
author_rank: Super Contributor
timestamp: October 17, 2025 01:38:31 am
content: Muting on should have no signal coming into V7A but if V7B was doing anything (noise) it gets over to V7A (via joined cathodes) and the noise would appear on C61. So this aspect can be confusing.For the phase-splitter, any signal to either V7A or V7B (grid) gets over to the other one, because of cathode coupling, they are tied together.It makes troubleshooting difficult.You can (AC) ground C56 or C58 or both, to stop signal coming in using a big cap like 0.22uFComparing voltages on V7A,B will also show health of that stage, they should be close esp. plate voltage. edit: (but plate resistor R107 82k is lower than R108 91k)

author: Terry Bites
author_rank: Super Contributor
timestamp: «Reply #16 on:Todayat 10:23:40 am »
content: As long as it doesnt affect the bias of the output valves it's ok.The op stage will be ac coupled, so no worries there.You need Mr Carlson's super probe.https://www.instructables.com/Carlsons-Super-Probe/