Gross loudspeaker cone movement when using tube amps.

[colinB]

Hi guys,

I'm relatively new to the world of tubes having only previously diy'ed a pre-amp (Simple 5687 DIY Paradise amp) and a triode strapped SE EL34 power amp.

This weekend I completed the Morgan Jones Bevois valley Ultralinear EL84 Push Pull amplifier which kicked off my current problem which I would appreciate some help with.

Firstly some background. I built the EL84 PP as per Morgan Jones schematic using Hammond iron 370HX + 1608 output tx's (similar to Kofi's build) except I changed the gridstoppers on the E88CC from 330R to 4k7 (high but all I had to hand) on both pins 2 & 7. This appeared to overcome all the stabilisation issues.

I tested this amp using a MP3 player connected directly and all was well except I had to use nearly full volume on the MP3 player due to the low sensitivity of the EL84 Push Pull amp. OK I was happy at this stage as I can 'simply' alter the feedback resistor/cap set-up to increase gain as suggested on an earlier thread.

Anyway, as I already had the 5687 pre-amp available I decided to use this between the MP3 player and the EL84 PP amp. This worked fine with volume control at ~12 o'clock but this was not unexpected.

However, when checking the amount of background noise (no music playing) I noticed that the loudspeaker cone was intermittently flapping very excessively so much that I quickly turned the amps off fearing damage to the drive units
On further investigation, I discovered that this 'flapping' only occurred when the pre-amp was connected in the chain. With just MP3 player & EL84 Push Pull amp connected together (no music playing) or EL84 Push Pull amp switched on with no source connected the loudpeaker cone didn't move at all.

By the way my loudspeakers are DIY using the Fostex full range driver FE167E in their recommended bass reflex cabinets without using any crossover.

I guess the excessive movement is due to some DC voltage on the output to the loudspeaker but I don't understand why it is happening only when the pre-amp is connected? BTW I need to use the pre-amp when I'm playing vinyl so I will need a solution.

I gave up on trying to fix this a few hours ago as I'm pretty clueless about the technicalities of tube amps so hopefully someone here can suggest a really simple solution which a beginner like me can implement.

Any help is appreciated

Colin.

[Original Burnedfingers]

What size capacitor are you using to parallel the cathode resistor?

I believe the circuit calls for a 1000mfd cap. You might try a larger cap. I had the same problem with a 12B4 line stage I built. The problem was solved with a larger cap.

[ErikdeBest]

Hi Colin

I understand your feeling quite well (I also lost weekends troubleshooting things), so, I will post something for you to reflect upon, but surely other people will jump in!

Looks like motorboating (valve amplifiers p410-411). It is just weird, as this problem is mostly caused by feedback through the power supply, but I suppose you are using separate PS for the pre with 5687 and the Bevois Valley power.

If you can not solve the problem, I recommend you to describe the PS arrangements you made!

Good luck, Erik

[SY]

If I understand your description of the problem, you've got a grunting 5687 preamp. Stick a scope on its output and see if it's flapping. A schematic would help, if you've drawn one.

[colinB]

Hi,

Thanks for the replies.

To answer the questions raised.

What size capacitor are you using to parallel the cathode resistor?

Burnedfingers - The capacitor on cathode resistor of 5687 is 470mfd bypassing a 470R. (See attached schematic)

If I understand your description of the problem, you've got a grunting 5687 preamp. Stick a scope on its output and see if it's flapping. A schematic would help, if you've drawn one.

Sy - Your description of a 'grunting pre-amp' sounds about right! I'm afraid I don't have an scope available only a DVM so I guess solving the problem maybe somewhat harder than it should be. Schematic for the 5687 pre-amp is attached (taken from DIY Paradise website.)

My power supply for this pre-amp is the same as the schematic except I used a 350-0-350 power trafo (Hammond 373X) & 5Y3 rectifier valve. Also (but probably not relevant here) I switched to DC heater supply for the 5687 due to hum. Only other changes 0.1uF caps were put on the input RCA's.

One final point I don't recall the loudspeaker cone flapping when I connect the 5687 pre-amp with my EL34 SE power amp.

Cheers
Colin.

[Sonusthree]

Originally posted by SY
If I understand your description of the problem, you've got a grunting 5687 preamp. Stick a scope on its output and see if it's flapping. A schematic would help, if you've drawn one.

I was wondering ...... would it be appropriate, since no 'scope is available, to connect the preamp to a soundcard running TrueRTA or similar? Or is it too risky? How about using test signals and a DMM to check for high voltages first?

I'd be really interested to know IF it's a good idea or not.

Regards,
Martin

[SY]

If the problem is very low in frequency, it will be tough to see on a sound-card. From the description, it seems like it's 2-4Hz and probably shaky. A scope is better for this sort of thing.

Leaky cathode bypass might be the problem- you can diagnose it by removing the cap altogether (this will drop gain) and seeing if the problem remains. You could also be suffering from line voltage variations, which will not be well rejected by a passive filter, and will also make the heater voltage bounce around a bit.

The general question of sond-card input protection is a good one; I tend to err toward the scared-to-death side. Between the DUT and the sound-card, I'll use a buffer which will limit the voltages seen by the sound-card. You need something like that anyway or the low input Z of the sound-card (typically 10k) will load down the circuit you're trying to measure.

[Sonusthree]

Cheers SY, much obliged! That's another lesson learned.

Martin

[zacster]

Could it be a microphonic tube or tubes in the pre? I had this happen to me using 5963s in my Foreplay. When I put in some new JJ 12AU7s it stopped. Always worth a try if you have another set of tubes.

[richwalters]

Originally posted by colinB

I tested this amp using a MP3 player connected directly and all was well except I had to use nearly full volume on the MP3 player due to the low sensitivity of the EL84 Push Pull amp. OK I was happy at this stage as I can 'simply' alter the feedback resistor/cap set-up to increase gain as suggested on an earlier thread.

However, when checking the amount of background noise (no music playing) I noticed that the loudspeaker cone was intermittently flapping very excessively so much that I quickly turned the amps off fearing damage to the drive units
On further investigation, I discovered that this 'flapping' only occurred when the pre-amp was connected in the chain. With just MP3 player & EL84 Push Pull amp connected together (no music playing) or EL84 Push Pull amp switched on with no source connected the loudpeaker cone didn't move at all.


I guess the excessive movement is due to some DC voltage on the output to the loudspeaker but I don't understand why it is happening only when the pre-amp is connected? BTW I need to use the pre-amp when I'm playing vinyl so I will need a solution.


Colin.

Any chance of a circuit diagram ?

richj