Tremolo Help

[martin manning]

To be clear, there is no problem with the Ampeg circuit HH has used, but there will be a Vh-k issue if a Fender circuit (with the CF driver) is built using a 6SLL7.

[Phil_S]

Martin, I am in awe of your capacity to reduce this to one line. Bravo!

[C Moore]

Is the board made of that black papery stuff by any chance?

Sorry...I missed this earlier.
No...it is glass...yellow turret board. I bought it from either CE or Ken at Turret Board.

[C Moore]

As I previously pointed out, the max heater-cathode voltage for a 6SL7 is spec'ed at 90V. The Fender circuits use the oscillator to drive a cathode follower stage which has no plate load and a large cathode resistor. This means a high cathode voltage, as much as 250V at the 400+ volts used for the supply. By the way, what is your supply voltage?

I am not sure anymore.
I believe I pulled it from the screens. And I Think that was about 320VDC.
I would just turn it on and check...but I still have it all torn up.
Let me get this all back to "normal" and I will give you the real number then.
Thank You

[tubeswell]

The LFO circuit with 3 sets of RC in the network has to have a gain of at least 29 according to this bloke http://www.valvewizard.co.uk/trem1.html

So that is a fully-bypassed centre-biased 12AX7 or 5751 with sufficient tube current to get as-large-as-possible signal swing. A fully-bypassed properly set up 6SL7 or 12AT7 or even a 12AY7 will also do this to differing extents, but the h-k voltage insulation rating isn't high enough to be able to reliably use the other triode as a CF buffer stage. Unfortunately, modern (New-Sensor) 12AX7s aren't reliable enough in this regard either.

However I have built a few 5G9s and used NOS 12AX7s in the 'trem tube' slot without an issue. Alternatively using a high-voltage mosfet (i.e. IRF820) source follower as a buffer gets around the h-k insulation issue and makes no difference whatsoever to the tremolo waveform (as this comes from the LFO tube stage).

The ampeg circuit ought to work, and the reason yours doesn't sounds (from all your reports) like an intermittent fault with the wiring. If you have a glass board it should provide adequate voltage insulation. Is the soldering clean? Is there flux residue between the eyelets/turrets? Is something on the back of the board shorting to something that it shouldn't be shorting to?

[C Moore]

Thank You -
I just printed that info from Merlin.
To the naked eye...with my reading glasses on...all really looks pretty good.
Before I put this headache back together...I will grab a magnifying glass and try to rule out any of your mentions.
Maybe when I get this back together...I will have magically fixed a problem that I was not aware of.
BTW...you mention the modern day Tung-Sol.
I blew up 2 of those in a real short time...in a Marshall...in the CF position.
Contacted the tube dealer...and he said (4-5 years ago) that he had just become aware of the problem.
Anyway...I have a doctor to go to...will get back to this in a few hours.
Thank You

[martin manning]

JJ's 12AX7 (ECC83-S) data sheet says 180V Vh-k max, and that is probably as good as any new tube. EH's sheet says 100V, and this would presumably apply to the other New Sensor 12AX7's. The Tube Store, however, is offering the old Tung-Sol USA data sheet (with Vh-k max of 180) along with their listing for the reissue, which is clearly wrong.

[JazzGuitarGimp]

I wonder about this sort of thing in the event I ever sell an amp. Does one just put a big sticker on the chassis, next to the tube, that says "Do not remove this tube! EVER!"?

[Firestorm]

I'd go NOS if possible, but I've had luck with some 12AX7s from Shuguang in old Twins Bassmans and Marshalls. No guarantee there since they seem to change the recipe at the drop of a hat. Whenever I build this into something new (or sometimes in something old where originality isn't a consideration) I elevate the heaters.

[martin manning]

That's my strategy too, although you can only raise them about 80-100V before you start getting into trouble with the other stages where the cathode is close to ground. That will cover the 100V Vh-k max tubes in most circuits, but it's not enough to run them in extreme cases like the 5G9.

[cbass]

JJ's 12AX7 (ECC83-S) data sheet says 180V Vh-k max, and that is probably as good as any new tube. EH's sheet says 100V, and this would presumably apply to the other New Sensor 12AX7's. The Tube Store, however, is offering the old Tung-Sol USA data sheet (with Vh-k max of 180) along with their listing for the reissue, which is clearly wrong.

Those old tung sols are very underrated(not cathode to heater) tubes.I think they sound fantastic.I like them better than mullards in a lot of amps(fenders).They can be Noisy(long plates).I'm not sure how many variations there are.But I think the ones I have are just Plain 12ax7 not !2ax7a.I'd have to look to be sure

I like them in the PI of higher gain amps.
Did I go OT again Sorry

[Firestorm]

That's my strategy too, although you can only raise them about 80-100V before you start getting into trouble with the other stages where the cathode is close to ground. That will cover the 100V Vh-k max tubes in most circuits, but it's not enough to run them in extreme cases like the 5G9.

The 5G9 looks like it should never have worked (for long).

[tubeswell]

That's my strategy too, although you can only raise them about 80-100V before you start getting into trouble with the other stages where the cathode is close to ground. That will cover the 100V Vh-k max tubes in most circuits, but it's not enough to run them in extreme cases like the 5G9.

The 5G9 looks like it should never have worked (for long).

But they work fantastic! (if you can find the right tube)

FWIW re-use those old microphonic NOS 12AX7 in the LFO slot in a 5G9. They work fantastic. Good recycling of otherwise useless tubes

[C Moore]

OK -
Let me ask one more (I hope) question.
It was difficult to build this amp...because of the space constraints.
I had to add a couple of turrets (eyelets actually) to the board...and had to attach one of the Power Supply caps under the board to even get it all on there. There was No Room for the bias circuit...so I put that on "top" of the chassis.
You cannot pull the pots off the chassis...if the board is in place. So...I am having a real hard time even getting this back together...and feeling any kind of comfort with the build quality.
I think it will be better if I...kind of...start over. I have space on top of the chassis to mount the trem circuit. Then...I can get all other components on the top of the board...and probably have a more intelligent lead dress...and more importantly...a much more serviceable amp.
So with that in mind...what do you guys think...should I adapt the chassis to take an AX7 type of tube...and change the trem circuit...or should I stick with this Ampeg design with the 6SL7.?
I am using a LTP for the PI circuit. It was WAY back at the beginning of this mess. Maybe some are under the impression that I was also using that "floating" PI as was in use with the Ampeg amp.
If I am going to reconfigure this...would I be wise to change to an AX7.?
Thank You

[C Moore]

OK -
Let me ask one more (I hope) question.
It was difficult to build this amp...because of the space constraints.
I had to add a couple of turrets (eyelets actually) to the board...and had to attach one of the Power Supply caps under the board to even get it all on there. There was No Room for the bias circuit...so I put that on "top" of the chassis.
You cannot pull the pots off the chassis...if the board is in place. So...I am having a real hard time even getting this back together...and feeling any kind of comfort with the build quality.
I think it will be better if I...kind of...start over. I have space on top of the chassis to mount the trem circuit. Then...I can get all other components on the top of the board...and probably have a more intelligent lead dress...and more importantly...a much more serviceable amp.
So with that in mind...what do you guys think...should I adapt the chassis to take an AX7 type of tube...and change the trem circuit...or should I stick with this Ampeg design with the 6SL7.?
I am using a LTP for the PI circuit. It was WAY back at the beginning of this mess. Maybe some are under the impression that I was also using that "floating" PI as was in use with the Ampeg amp.
If I am going to reconfigure this...would I be wise to change to an AX7.?
Thank You