VOM 8810 Project

[Platefire]

Yeah, I was just wondering about the switch first in line but-----no problem putting the fuse first. I got parts ordered for this project and just trying tosort a few things out in my mind before I start building the stripped down chassis back---Thanks!

[Platefire]

The NFB resistor on a 5F1 and a 5F2A are both 22K and as I understand it both had 4 Ohm speakers. This little booger has only an 8 Ohm OT tap

I have read that a 33K would give you approximately the same amount of NFB with a 8 Ohm speaker as the Princeton/Champ with 22K and 4 Ohm speaker.

Is this correct??

[sluckey]

Close enough.

[Platefire]

Thanks sluckey!

[Platefire]

Another question!!! On Output Transformer continuity?
All directions on measuring Ohms on OT continuity indicate should be low readings but don't indicate what Ohm range they consider low.On this OT the reading between secondary's is 1 Ohm and between primary's is 290 Ohms.
The 290 Ohm seems more than low. Is that more than it should be?

[kagliostro]

Consider that in an OT what is important is the turn ratio, resistance of the windings is all one other affair

the way the windings of the transformer are realized changes the resistance of windings also if they has the same number of turns because the windings can be of different lengt (turn on the core 200 coils and then turn over that 200 coils other 200 coils, turn are the same but the lenght of the second 200 turns is more because they are wrapped on a bigger diameter)

and you must consider also that two transformers that has the same turn ratio if planned for different power are realized with wires of different thickness and this result in different resistance

So your question seems to be impossible to have an answer, the only thing you can do is to measure one other OT of the same kind ad compare the result

Franco

[sluckey]

On this OT the reading between secondary's is 1 Ohm and between primary's is 290 Ohms.
The 290 Ohm seems more than low. Is that more than it should be?

Sounds typical to me.

[Platefire]

Good! Getting close to start building it back, so checking everything out I have questions about first. Thanks!

[PRR]

If you don't know better, pretend the DCR is 1/5th to 1/20th of the nominal audio impedance.

Large DCR wastes audio. Small DCR wastes money. So 1/10th is good, with wide variations.

Then 290r is spot-on for a 3K audio winding.

Large narrow-band guitar OTs often have quite low DCR for their impedance.

[Platefire]

Har, I'd definitely come in the "don't know better" group. Impedance is something I've never been able to wrap my head around. I can see high voltage being fed though the primary side of the OT to the tube plate and like a capacitor the high voltage is blocked and the low voltage AC is allowed to pass through the secondary windings to the speaker. Also through the secondary windings the different taps give you 4, 8 and 16 Ohms taps. This is as much as I know about OT's. I realize that it's all based on formulas and if I do learn some of it, by the time I need them again, I've forgot and have to learn it all over again. If I used it regularly, it would be a lot easier to remember

Just looking for some good info on the subject I found the 1960 article that seems pretty good:
https://pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_06/Sec_26/1555_Output_Transformers.pdf

[Platefire]

I'm still hung up on this OT.

So PRR, please tell me how you determined this is a 3K Primary winding? Also that concerns me because my Allen OT on my DIY Champ has a 8K primary winding.

so when you say "DCR" I'm assuming that is the resistance between the anode on the primary side of OT?

So I'm trying to understand why the difference between the VM 3k/8 Ohm and Allen 8K/8 Ohm with both being 6V6 output tubes

[shooter]

the OT is a step down transformer
put a small AC voltage across the secondary windings, measure the "stepped up high voltage" on the primary
calculate the turns ratio


3K/8 implies you get a 3K load because you have a 8 ohm load on the secondary, if the secondary has a 4 ohm load you now have a 1.5K load on the primary.
 

[Platefire]

OK, all I had was a AC transformer that put out 13.90 VAC. Applied to the secondary's I get 331 VAC. 331 divided by 13.90=23.81. So that's 23.81:1 TR


So 23.81 squared is 566.91.  6V6 ELR=8000 Ohms/566.91=14.11 Ohms Speaker Impedance

Have I got it right or did I make an error? Don't calculate out close to 8 Ohms for sure!

I think the 8000 ELR is for a PP 6V6. If SE 6V6 were 5000 ELR, it would calculate to 8.8 Ohm speaker Load

[PRR]

....how you determined this is a 3K Primary winding?

I didn't. It isn't. Looks like I mixed this with another thread.

4.5K is a fine load for single 6V6.

[Platefire]

Well PRR it's good to know I'm not the only one that gets one tread mixed up with another
OK, good on the OT. For some reason I was really questioning it but now I'm feeling better about it. We'll let the OT now rest in peace

[Platefire]

This little chassis, crowded for space, has me thinking about things I normally wouldn't have to consider in a larger space. So I would rather head off any noise makers in advance if possible. So questions:

I got rid of the heater balance pot and am going to create a artificial center tap for the heater wires using 100 R's. So my question is there any potential of creating noise if I ground 12AX7 pin 3 cathode Resistor and bypass cap on same ground a artificial heater center tap ground??? or do I need to create a separate ground point for my 12AX7 cathode?

Also it looks like I will running the heater wires to the pilot light in close proximity to the 6V6 Heater wires and also main power Cord AC leads. Any potential noise with that?

[shooter]

I ground 12AX7 pin 3 cathode Resistor and bypass cap on same ground

I try and keep heater CT in the "dirty ground" box, so I terminate at chassis or with PA grounding.

[Platefire]

On the cathode ground, I can get a chassis ground from a less convenient source, just have to run a little longer ground wire. Thanks

[Platefire]

Another Grounding question??? On my 6V6 Power tube. I'm installing a terminal strip to ground the cathode resistor and 25 uf bypass cap.

I was wondering if it would be a bad idea to run the 6V6's pin 5, Grid to ground 220K resistor to the same ground on the  terminal strip?

I'm being very careful because when I put this little crowded buddy back together, it won't be easy to tear it back down to correct mistakes. Thanks, Plate

[shooter]

this was my last build, I wasn't being carful, since it was "built" as a tinker'n amp, to hack up many times.
No problem with the grids grounded off the terminal strip using either 6V6 or 6K6

[Platefire]

OK, Shooter. Thanks! Does that happen to be that 1482 variation you built a while back?

[shooter]

yep, although the pic is during the conversion back to 6K6, (Blew a 6V6 n no spare), also modded as an Audio amp as my "center channel"

[JASO410]

This little chassis, crowded for space, has me thinking about things I normally wouldn't have to consider in a larger space. So I would rather head off any noise makers in advance if possible. So questions:

I got rid of the heater balance pot and am going to create a artificial center tap for the heater wires using 100 R's. So my question is there any potential of creating noise if I ground 12AX7 pin 3 cathode Resistor and bypass cap on same ground a artificial heater center tap ground??? or do I need to create a separate ground point for my 12AX7 cathode?

Also it looks like I will running the heater wires to the pilot light in close proximity to the 6V6 Heater wires and also main power Cord AC leads. Any potential noise with that?

I have been stuck on this exact same conversion for like 2 months now. Got everything squeezed in there and it fires up. However I get only a small amount of output when running an audio generator through it. Nothing with the guitar. The volume control doesn't seem to function. Planning on going back to it here soon. It's just such a headache to unsolder everything and resolder it again that I have been unmotivated. Hopefully you have better final results. I did a 5f1 champ style build with Merlin's suggested modifications to limit bass and his 10k input jack.

[kagliostro]

.... Merlin's suggested modifications to limit bass ....

Don't remember were, can you remember it to me ?

Thanks

Franco

[JASO410]

.... Merlin's suggested modifications to limit bass ....

Don't remember were, can you remember it to me ?

Thanks

Franco

https://ibb.co/pr2249c
https://ibb.co/87q89M4
Where are two imgbb links from Merlin's book that will demonstrate what I mean. Hope this helps. -cheers
Jaso

[Platefire]

Yelp! what a challenge on this little buddy. I'm now finally making progress on the build back better plan:>)
It is so important that you build it back in the right order as to not to block access to other parts that will have to be installed latter,  so I wrote out a two page build back plan to help prevent me from doing that. I'm taking it slow and methodically as to hopefully get it right? it will truly be a little scary on the first fire up

[sluckey]

Now that's the kind of chassis that I'd just throw away. Harvest the transformers and build in a fresh proper chassis.   

[shooter]

build it back in the right order as to not to block access to other parts

worked on many a system in my career where the mechanical Engineers didn't seem to get the memo "Hey, that part you buried in the bowels of this thing might break some day" or they were just smoke'n in the boyz room 

[Platefire]

Yelp! Live and learn. I always thought these VM 8810's were cute but they don't quite have the appeal they use to. Still Praying for good success!

[Platefire]

Got a question on Shielded Cable. I was planning on using some vintage shielded cable I got from a Hammond PR-40 Tone Cabinet because it was more heavy duty than the RG174. So when I started making up a run to go from the input jack to 12AX7 pin 2, I noticed that the shielding didn't cover the complete outside area but was like a spiral staircase with a gap in between runs.

When I saw that, my thoughts was "This is not true shielded cable" I made my other two runs using RG174.
Before I toss the first run I made with the vintage cable, are my assumptions right regarding this cable? I'm asking because I have a lot more and want to know for sure:>)

[shooter]

it should be fine.  shielding works by "offering" noise a better "short" to ground than the center conductor path through it's travels

[PRR]

Huge gaps have little effect. Hammond was not so dumb.

[Platefire]

Well I 've had a problem I've  ever had  before!
Getting solder in the pin sockets in my Belton 9 pin. I think it's because the odd angels I'm having to solder in this little chassis. No other way to solder with gravity flowing toward the pins. I couldn't repair it so now its been replaced with another Belton.


So now I've been reading up on how to prevent this because I surely don't want to repeat this. I have read the most common practice is to install a dead 9 pin tube in the socket when soldering terms.
Of course the most important is Good soldering practice. Any other suggestions welcome!

[sluckey]

If you are having problems with solder running down into the socket pins, you may be a red using too much solder.   

[shooter]

 
I might resemble a red, but i've always been pretty good with a solder pencil.  Had the same problem on the white ceramic sockets, the one I smashed up with a hammer had a "wick-hole" at the base of the pin cups, so when you touched solder to the tab, slurpyed straight up the hole!!


Did what you did but added High voltage grease to the tube pins, then Alcohol them clean afterwards.  The sockets I bought were from a surplus electronic place, pitched them all!

[Platefire]

Well, I'm 'sure part of the problem is I've got in the habit of using a old Weller S 200 N gun for everything.
Probably need to switch to a low Watt pencil iron
on this.


Never heard of high voltage grease??

[shooter]

Weller S 200 N gun

ya, that might be a red thing!
i use a Weller 25W pencil with either a 1/16" or 1/8" chisel bit


used HV grease A LOT. you goop up the 140KV "candle-sticks" that plug into an X-ray tube, keeps the 3 pins on the sticks from arc'n between pins.  It's a special type of silicon, use it whenever I pull off connectors under the hood, keeps moisture out.  The stuff NAPA sells "dielectric compound" is like $10 for a small tube

[PRR]

> a old Weller S 200 N gun for everything.

And the 20 pound sledge to hang pictures on plaster walls? The axe to trim your toenails?

I wish I could tell you to get a good Weller 35W iron. Sadly the cheap stuff is all crap (or $65) and you won't do enough fine work to need a "Soldering Station".

[Platefire]

The Weller gun has a trigger for lower Watt burns. You don't have to run it wide open all the time. I like the extra power there when you need it.


I do have several pencil irons but they of RS verity. Will be super careful this go around

[AlNewman]

I bought one of these about a year ago.  Seems to work pretty good, have used it lots since then.  Cheap like borscht.

https://www.newark.com/xytronic/t-1976/60w-temperature-controlled-soldering/dp/98Y9882

[shooter]

Weller gun

^^^that plus your fingers wrapped around the handle, trying to solder inside a small chassis, it's real hard to see, get the right angle, feed solder
it's the 'ol adage about right tool for the job thing

[PRR]

> The Weller gun has a trigger for lower Watt .....I like the extra power there when you need it.

I have several Weller guns; I know how they work. Even 140W is too much inside a chassis, EXcept for soldering to the chassis itself. Leaving out that chore, my 45W straight-iron will over-cook anything in electronics, and even #12 busbar. I guess last time I had a Gun out was a 60-Amp fusebox for the backhoe.

[WimWalther]

I used the Weller WTCP type irons (700F or 800F tip) for almost 20 years. And while they are a good, fast-working iron, I eventually got sick & tired of having to repair the pencils, which aren't much fun to work on, diagnose or buy parts for.

A friend gave me a couple Hakko 926 stations (900 pencil) and I've never looked back. The Hakko system is far more reliable, and they have a temp adjustment control.

If you can pick up just the 926 base station cheap, Chinese knockoff 900 pencils can be bought new for around $15 complete. I've been using one for 5 years with no problems. The cheap knockoff tips are fine too.

[johnnyreece]

I picked up a Xytronic soldering station years ago (still had the analog temperature select; new ones are digital), and I think it was only $65 or so.  It's served me well for a decade plus.  Not sure about current quality on them, but mine's been fantastic. 

[Platefire]

Thanks! I'm well ready to amend my ways to help my soldering situation especially on this project. I think a good 20 watt pencil iron would do since I don't do a lot of soldering any more. I got a couple of pencils but I don't know how good they are. I'll have to dig them out and see. I probably need to do some practicing on some old parts to get the feel

[Platefire]

UH OH! Impulse Buy. I dropped in to Harbor Freight yesterday to see what kind of soldering irons/stations they had.They had this Schneider 5 to 50 watt station, Harbor Freight brand on sale for $40.00. So before I knew it, I had purchased it. So after the fact I have been reading reviews on it. I haven't cracked the box yet. I guess it's what you would call your very basic station

[Platefire]

Well I finally cracked open the box to the soldering station. Read the instructions, hooked it up and tried a practice run on the previously destroyed tube socket. It went so well I went ahead and soldered back the disconnected connections that I had made on the removed destroyed socket. Looks like it might be pretty good.

One thing I'm wondering about though is I thought the melting temperature would be between 200 to 300 degrees.I tried that range and it would not heat up a joint to melt solder. I had to turn it up to the 700 degree mark to do the job. I'm using 60/40 Rosin-Core, .050 Diameter. Is that normal?

[shooter]

I believe the flow temp is in the 600 degree range, 700 is "normal" in my world, my solder is .8mm, you'll have to do the math 

[Platefire]

Yeah, .8mm=.031 Inches. From my study on the subject, I'm using a little bit larger Dia than needed for what I'm doing.

Think I'm ready to re-start this project after this little set back:>)

[PRR]

"Make sure the surfaces to be soldered are clean. Turn on the soldering iron and set temperature above the melting point of your solder. 600°- 650°F (316°- 343°C) is a good place to start for lead-based solder and 650°- 700°F (343°- 371°C) for lead-free solder."
https://www.techspray.com/ultimate-guide-to-electronic-soldering

So do you do C or do you do F? And what is the station marked in? Each degree C is bigger than a degree F so you need fewer C degrees.