tweed vibrolux

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tweed vibrolux

Postby centervolume on Wed Feb 10, 2016 10:54 pm

Is it true that there are NO 1MEG resistors on the input jacks? Doing a build off the schematic and this seems to really stand out in comparison to almost every other schematic. Dont have an original to compare it to, wondering it it was a typo or something ?
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Re: tweed vibrolux

Postby pdf64 on Wed Feb 10, 2016 11:40 pm

It looks like the input stage relies on whatever is plugged into it to provide a path to 0V, which was probably a safe bet in 1957, and still is today.
However, if it is lost, the tube's operating point may drift off, and it may even redplate!
It would be better if eg 1M was fitted across the muting switch contact of input #1.
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Re: tweed vibrolux

Postby centervolume on Thu Feb 11, 2016 3:20 am

There are the 68Ks on the board but that's a pretty far cry from 1MEG.

I have adopted that front end for a build I just wired up, starting to work out the issues.

The main one being motor-boating whenever anything is turned up (tone controls or volume). And that's with nothing plugged in/ the inputs are muted. I just wired it up and turned it on for the first time today, so these issues are somewhat to be be expected given this is my 3rd build and 1st time from a custom schematic I want to realize. wish me luck!
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Re: tweed vibrolux

Postby Unit_1 on Thu Feb 11, 2016 6:01 am

:highfive: LUCK! :highfive:
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Re: tweed vibrolux

Postby pdf64 on Thu Feb 11, 2016 9:08 am

Yes, good luck :fingersx:
Motorboating often indicates inadequate decoupling on the HT, eg too many stages sharing the same node; or maybe just left the 0V connection off a HT cap kinda thing!
Let's have a look at your schematic?
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Re: tweed vibrolux

Postby PA100 on Thu Feb 11, 2016 11:03 pm

I got motorboating when I recapped my fender.....i was changing all coupling caps from 0.01 to 0.1 uf for better bass. Then I had motorboat sound... I thought it may have been a resistor change I did as I changed a wrong value resistor on the reverb circuit. When I changed it back the motor boating changed speed... So I knew I was in the right place but wasn't the resistor... The resistor connected to the reverb coupling cap . I reverted to the .01 call and it was fixed. My thoughts are that possibly you have too higher value cap in a similar position? Maybe even decimal point on schematic could be in the wrong place.
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Re: tweed vibrolux

Postby pdf64 on Fri Feb 12, 2016 9:36 am

Yes, in regard of motorboating it's a balance between signal path coupling and HT decoupling.
To maintain stability, if the signal coupling is moved down, the HT decoupling should be increased commensurately.
Bear in mind that it's probably counterproductive for the amp to try to put out frequencies that the speakers can't reproduce, as the amp may get pushed into overdrive and everything gets turned to mush.
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Re: tweed vibrolux

Postby centervolume on Sat Feb 13, 2016 6:50 am

I got the motorboating straightened out; ended up sounding like motorboating when it was insecure grounding at input jacks.

it passed signal for the first time tonight, its a groovy little monster (cathode bias, class A with 5881 and toggle to activate 2nd 5881 in case of emergency, has full assortment of treble bass mid controls [5f8-a tone stack] all in a tweed vibrolux box w/ alnico 10, 20 watt OT)

I do have what I think/hope is my final problem - I have a residual hum in the power tube section. I suspect it is because I grounded all of the following to a single point:

1. filter caps
2. red/yellow PT center tap
3. power tube cathode bias assembly (this is the one I currently suspect as needing to be separated given its proximity to signal... does this sound
like something I should ground to another location?)

it sounds like a 60 cycle hum so I double checked my heater pairs, they hit the same pin #s on both 5881s and the same ones on the 2 ax/ay7s (I don't recall if they are supposed to alternate for example pins 4/5 on v1 go to pin 9 on v2).

I don't have a choke installed.

The heater has a center tap (green yellow) that I soldered to the inside wall of the chassis with my 80 watt solder canon.

Got to nail down that blasted hum!!
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Re: tweed vibrolux

Postby pdf64 on Sun Feb 14, 2016 11:37 pm

The reservoir cap is best being connected directly to the PT HT CT, as that's the highest level of dirty current.
Then that connection be referenced to the power amp 0V common (along with eg the power tube cathode current).
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Re: tweed vibrolux

Postby Dean Jr. on Mon Feb 15, 2016 10:52 pm

centervolume wrote:I got the motorboating straightened out; ended up sounding like motorboating when it was insecure grounding at input jacks.

it passed signal for the first time tonight, its a groovy little monster (cathode bias, class A with 5881 and toggle to activate 2nd 5881 in case of emergency, has full assortment of treble bass mid controls [5f8-a tone stack] all in a tweed vibrolux box w/ alnico 10, 20 watt OT)

I do have what I think/hope is my final problem - I have a residual hum in the power tube section. I suspect it is because I grounded all of the following to a single point:

1. filter caps
2. red/yellow PT center tap
3. power tube cathode bias assembly (this is the one I currently suspect as needing to be separated given its proximity to signal... does this sound
like something I should ground to another location?)

it sounds like a 60 cycle hum so I double checked my heater pairs, they hit the same pin #s on both 5881s and the same ones on the 2 ax/ay7s (I don't recall if they are supposed to alternate for example pins 4/5 on v1 go to pin 9 on v2).

I don't have a choke installed.

The heater has a center tap (green yellow) that I soldered to the inside wall of the chassis with my 80 watt solder canon.

Got to nail down that blasted hum!!


So...this amp is actually NOT a tweed Vibrolux but rather might be closer to a Gibson GA-18 or something with a swtichable-paralleled single-ended output, right....with a full TMB tone stack...sort of a 'true dead ringer for something like you ain't never seen before'???
Kudos on getting it sorted out and running.
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Re: tweed vibrolux

Postby centervolume on Tue Feb 16, 2016 3:31 pm

that's right...

my cathode bias on the 5881s is going nuts right now, spiking at .15 up .2

it's coming along
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Re: tweed vibrolux

Postby Dean Jr. on Tue Feb 16, 2016 8:13 pm

Forgive my density....''spiking at .15 up .2" regarding the bias. What does this mean?
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Re: tweed vibrolux

Postby centervolume on Wed Feb 17, 2016 6:40 pm

not at all, my terminology - and even what I am getting on my measurements - are out of the ordinary.

when I measure amps power tube bias 99% of the time. my meter gives me something in the .020 to .035 range for normal operation (.065 or higher for cathode bias). For the normal / PP tubes, I tend to refer to that as "twenty to thirty-five milli amps."

In this situation, I got some initial readings about where I expected for cathode bias, namely 70-90 milli-amps, then it went up above .15 towards .2. Those are what I am perceiving as spikes.

I dont have much experience with parallel single ended power sections, so I think the difference from PP is that I have to divide these apparently high figures by 2 (since there are 2 power tubes working as one, not in independent PP configuration). This would put them back to 75 milli amps per tube which is more what I was expecting
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Re: tweed vibrolux

Postby Dean Jr. on Wed Feb 17, 2016 9:17 pm

Okay...so...amps/milliamps. IN the above for fixed biased, you are used to seeing a range from .025 amps/25 milliamps
to .03amps/30ma. IN cathode biased you are used to seeing 65-90 milliamps...065-.090 amps. Your 'spikes' were .15-.2amps or 150-200milliamps. Yes, divide by two when using a common cathode bias resistor....so 75-100ma.
One would have to consider the plate voltage to understand what is going on as far as plate dissipation..which is the final number on wants to know when biasing....but the 65-90 ma figures, which is 37.5-45ma per tube, are to the low/cool side of things, imho.
However.....'spiking' is interesting. Do you mean that with the amp at idle the bias is fluctuating?? That can't be good.
something is incorrect....
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Re: tweed vibrolux

Postby centervolume on Sat Feb 20, 2016 4:17 am

no, not jumping up during operation. Just moving around as I get my power section values ironed out (filter caps, the 2-5 watt resistors in there etc). There are 400 volts on the plates.

So there isn't any problem (other than safety) with using the shunt method in a cathode bias setup with the parallel single ended power tubes? I know I have measured single cathode bias power tubes using that method. I just am not used to dividing by 2 which makes perfect sense.

here are the layout and schematic (hand drawn so there's that)

http://sclkssl.ssl.hwcdn.net/21/imgpages/1/4/4422021_775571.jpg?version=240

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