VOM 8810 Project

[Platefire]

This is a continuation of linked thread below. The guy I was trying to help do it himself has instead sent it to me to convert it. My thoughts are to convert it to a 5F2. So if your interested in previous history, here is the link. I decided to just start a new thread on it.

https://el34world.com/Forum/index.php?topic=30646.0

I just got the amp in and looking it over.  Schematic shown below. I have a question about the existing circuit. The schematic shows the PT Red secondary's leads routed to the rectifier as normal but---on the actual amp there are two 500 Ohm/10W resistors between those red leads and the rectifier. I never have seen this done before so I'm wondering what they for. Only thing I can thing off is reduce AC voltage to rectifier???? Anybody know what these might be there for?

Picture of the big resistors between PT red AC leads and rectifier is below.

[tdvt]

Those look like pretty modern Xicon power resistors in the pic.

Part of someone's earlier re-configuration..?

[Platefire]

He said that he replaced existing caps and resistors as was already in there. I sent him an e-mail to verify that but pretty sure he just replaced existing with new. This may be a change that VOM did that never got added to their schematics.
I'm pretty sure I'm going to do away with them in my conversion. I just never seen that before and thought someone could shed some light on it

[kagliostro]

Ciao Platefire

.... but---on the actual amp there are two 500 Ohm/10W resistors between those red leads and the rectifier.

Source
https://el34world.com/Forum/index.php?topic=20321.0

---

On Tube rectifiers datasheet you can find indications about those resistor between the PT and the Tube Rectifier



On this link you can read explanations about

http://www.valvewizard.co.uk/bridge.html

Franco

[sluckey]

on the actual amp there are two 500 Ohm/10W resistors between those red leads and the rectifier. I never have seen this done before so I'm wondering what they for. Only thing I can thing off is reduce AC voltage to rectifier???? Anybody know what these might be there for?

Look at this web page. Scroll down to the gut shot pic and read what's written below the pic.

     http://sluckeyamps.com/november/november.htm

Now you've seen another amp that did the same thing.   

[Platefire]

Thanks a bunch kagliostro for the info and the schematic showing the resistors.

sluckey thanks for you link to your amp project.

I have used big resistors to reduce voltage before but it was on the DC side after the rectifier. I never seen it done on the AC side of the rectifier. So based on what I'm understanding about it so far------at some point VOM must have realized their 8810 PT was putting out too much voltage and rather that changing the transformers, just added the resistors--or am I blowing in the wind

[kagliostro]

Read what is written on the link I posted

Those resistors may be added also depending on the spec of the PT, I don't mean the voltage, I mean the winding resistance

Franco

[Platefire]

Yeah, I have gone over it and bits and pieces make sense and others is just over my head. Like this statement under Valve Rectifiers makes sense but then the formulas is another thing:

 The data sheet will present table or graphs showing the minimum limiting resistance needed for a given application. If the transformer alone doesn't have enough resistance to meet this requirement then you need to make up the deficit by adding resistors in series with each anode. These resistors need to have a power rating that comfortably exceeds:
 
 P = (1.1 × Idc)^2 × R

[PRR]

I am surprised V0M would do it that way because the 300r in the negative lead of the first filter cap does about the same thing (limit cap-charging spikes).

OTOH I have seen the inside of some of these VoM chassis and it is possible even VoM itself could not figure out what went where. Or had a "finished" batch that they wanted to mod for better rectifier life (lower warranty costs) and it was easier to hack at the + side than the - side.

[PRR]

With about 1 Ohms, peak rectifier current is 3.2 Amps. With 300r, about 1 Amp. With 800r (500+300) about 0.5 Amp. That is the first half cycle.

Tube data for these types says 0.21Amps peak, but measuring peaks was not routine in those days. Still 3+A is just wrong (also impossible with real transformer), and 1A is maybe early-death instead of instant-death. So the added resistance may be needed for long life. The neg-lead resistor "should" be 300r because that is 6V6 bias. You need at least one more 500r. It "could" be designed into the winding, but maybe the supplier "improved" the design with fatter wire (more cost but less breakage).

Sim of intermediate value:  {redacted}

[dwinstonwood]

Sim of intermediate value:

PSUD3? How/where does one get that version? I don't see it on Duncan's site.
Thanks

[PRR]

> I don't see it on Duncan's site.

That was indiscreet of me. PSUD3 has been coming for many years. A very small set of testers have seen a beta.

[Platefire]

Thanks!! Well it's kinda hard for me to wade through all the technical jargon because of my lack of knowledge but just reading between the lines, I guess the two 500 Ohm/10W need to stay

Did some voltage readings on the existing and bear in mind, the voltage to the OT is not from the first power node but from the second after a 1K/5W R. Plus this is loaded readings with all the tubes in place. The newer canned filter
cap my friend installed is a 40/40/40 525V----so no graduated down uF nodes for preamp tube

Prior to the two 500/10W Current Limiting Resistors 381/381VAC

After two 500/10W    "           "            "                 366/366VAC

pin#7 6x4 rectifier-------------------------------------360VDC

OT Primary red----------------------------------------312VDC(after 1000/5W R)


This is it:

[Platefire]

OK, we have established the PT is too hot and needs to be toned down and the two current limiting resistors are justified. So from here I need to work out how to mod the existing power supply to work for a 5F2 Princeton circuit I'm planning. So your help is appreciated. Let me speak what I'm thinking and if I'm thinking wrong, please let me know.

1-The existing CE can cap is fairly new and unused but is 40/40/40 uf/525V. I'm thinking this is way to high uf especially for the 12AX7 pre-amp. I'm looking at a CE Can 40/15/10 uf/450V with also a 25/25 term. To me this is more in line with getting the correct 5F2 response without being to stiff? Also I could use the can 25/25 term for a V1a cathode bypass since I'm very limited on space in the little amp.

2-I've presently got 360VDC on pin#7 from 6x4 rectifier. The existing power supply has a 1k/5W resistor on the first  to second 40uf can cap terms reducing the voltage to 413VDC to OT. I'm thinking doing away with the 1K R so the 360V will go to OT. 

3-The normal 10K/3W and 22k/3W 5F2 resistors between power nodes may have to be adjusted to get the 12AX7 pre-amp stages plate voltage in the 200VDC range using the normal 100K Plate resistors.

Hope I'm in the ball park? Let me know

[kagliostro]

Consider that if you search you can find lot of discussion about increasing filtering on 5F1 & 5F2

an example

https://el34world.com/Forum/index.php?topic=14464.0





Franco

[Platefire]

Thanks Kagliostro

  Yeah I have had some good 5F1 Champ experience with my own DIY Champ. I originally built mine with the original 5F1 cap values. It was just an ever constant humm that you couldn't ignore. I lived with it a couple of years but finally did something about it. I did increase the values up some except for the 12AX7 and I left it 8uf. I was concerned about effecting the touch sensitiveness of the pre-amp. So I didn't use a high as value as you suggested and fortunately it was enough to get rid of the humm and not effect the touch sensitivity. I should have bumped up the pre-amp stage to at least 10 uf but on the other hand, it fixed my problem. Attached is a schematic of my DIY Champ that has a switchable 5F2 tone stack. So I am carefully considering just what to use on the VM. I was looking at the 5F2-A and noticed it had an extra filter stage prior to the power nodes and I noticed on your drawing you added another filter stage plus a 250-330R. I know the extra cap is for extra filtering but what is the 250-330R for?

I was just wondering about my existing 40/40/40 uf. Since it's a new can cap already installed. I wonder if it would be workable to use 40uf forthe first three nodes and install a separate single 10/500 axle for the pre-amp section. So instead of being 40/40/20 & 8/500, it would be 40/40/40 10/500---or is that too heavy filtering for this little amp?? Platefire

[kagliostro]

Ciao Platefire

.... I noticed on your drawing you added another filter stage plus a 250-330R

No, not my drawing, it is PRR drawing

... ... but what is the 250-330R for?

it is necessary to form a CR filter

here you can find documentation

http://www.valvewizard.co.uk/smoothing.html

---

Looking one other time to the 6X4 datasheet I noticed that the maximum allowed first cap value is 10uF, may be this is a further reason for the presence of the two 500R resistors

---

Usually if you need a high e-cap value as to filter in a better way B+ as to exceed the first cap value allowed, you install a first e-cap with the max value allowed on the datasheet, then a resistor followed by one other e-cap that can be large enough to satisfy the circuit necessities

So, if we would follow the datasheet to prevent problems to the rectifier you must add a first CR filter that uses a 10uF e-cap, this in theory, because as we can see on the schematic, the first e-cap value was stated to be 40uF

About this thing I would like to hear PRR opinion

---

If you want to add a 10uF for the preamp you can, but I'll try to use the cap you have as is

or

Assuming you want to ..... decrease the value of a cap on that Multi Cap Can (to feed the preamp) I think you can adopt a trick

If you put a capacitor in series with one of the capacitors on the Multi Cap Can you obtain that the Maximum Voltage affordable by the cap will be the sum of the voltage each cap can afford, if you have a 500V cap with a 500V cap in series the result will be a cap that can afford a 1000V B+, the counter part is that capacitance will decrease, two 40uF in series result in 20uF

So, assuming you want to tame a 40uF 500V cap to 8uF you must add an in series e-cap of 10uF

As your Multi Cap Can can afford 525V B+ and 525V is enough in the circuit, the added 10uF e-cap can be of very low voltage (also less than 50V), so it can be very small (remember to install 220K resistors in parallel with the in series e-caps)

Franco

[Platefire]

PRR, kagliostro and I are discussing proper filtering in my converting the VM 8810 to a 5F2. Kagliostro referenced one of your 5F1 drawings in previous post and wanted you to comment on it. So I'm converting the 8810 schematic below to 5F2 your drawing below. Will this RC filtering allow me to have a 40uf right after the 6x4 rectifier safely and tame ripple?

[kagliostro]

About the use of an in series capacitor to tame the capacitance I investigated furtherly and is possible but isn't possible to use a low voltage cap, so no benefit about the dimensions

Franco

[Platefire]

Well one thing I know is the power supply that is in there is apparently working and has a similar first 40uf reservoir cap but instead of PRR's 250-330R, there is a 1K/5W in that position. So that leaves two more 40uf terms on the can. So the next 40uf term is already the node for the OT, then 10K R, then next 40uf term to 6V6 screen, Then 22K to pre-amp and install a separate 10uf/450 axle for that after the 22K. My only issue with this it the voltage to OT with this arrangement is only 315VDC but if I removed the 1K and used PRR's 250R value, that would increase the voltage a little higher. This way I can keep the existing can cap and just add a 10/450 Axle for pre-amp.

I truly want to get this settled in a good workable plan before I dive in doing the other mods because space is tight and one thing can severely effect the other in this situation. Here is a pix of the can hookup situation presently

[sluckey]

Since space is so tight I would use this 40/20/20/10 can for the filter caps. You'll still need a small size 25µF/25V cathode cap.

https://www.tubesandmore.com/products/capacitor-ce-mfg-475v-40202010-f

[kagliostro]

This is an idea you can try using the Multi Cap you have

if it will not satisfy you I suggest to use the Multi Cap that Sluckey linked




For the 8uF you can use a small radial

Franco

[Platefire]

Hello! I've refreshing my memory and reading about how ecaps effect tone and touch sensitivity. The fine balance to try to straddle control of ripple/hum but at the same time not to much that would stifle the touch sensitivity and making response too stiff. So we know that the 5F1 and maybe the early 5f2 had hum problems from to low ecaps. Especially 8uf on V1. I know on my DIY Champ just moving the cap arrangement from 8/8/16 to 8/10/22 took care of my humm problem.

So the 5F2-A power supply as has been recommended with the addition of an extra 250/5W R between the first reservoir cap and first power node to OT, I am certainly agreeable to and I appreciate the help!!!

I think it may be more opportunity to have a more responsive amp with the can caps sluckey suggested. I may need to talk to my friend since he will be paying for that. He installed the 40/40/40 can and thinks it's fine. So I may need to explain what I'm trying accomplish and see if he buys in. It will no doubt work as Katlistro or sluckey suggested. With me, since I'm already in the process of converting the amp, I'd prefer to do everything I can to achieve the
best results first time out of the box
 

[kagliostro]

The decision is your's

If you want to try to be on the cheap and save around 40 $ you can give a try to the modded amp with the Multi Cap Can you have plus a separated one of 8uF for V1 node (I suggest a cheap modern small radial instead of an axial) and if it accomplish your expectation you finished

On the contrary, if the modded amp will result to be too stiff you can try to replace the Multi Cap  with the one that Sluckey suggested, hoping that it will give you the desired effect

Franco

[tdvt]

The decision is your's

If you want to try to be on the cheap and save around 40 $ you can give a try to the modded amp with the Multi Cap Can you have plus a separated one of 8uF for V1 node (I suggest a cheap modern small radial instead of an axial) and if it accomplish your expectation you finished

On the contrary, if the modded amp will result to be too stiff you can try to replace the Multi Cap  with the one that Sluckey suggested, hoping that it will give you the desired effect

Franco

I took notice over the last year or so as various sources kept mentioning the continuing demise of axial caps & I have been trying to implement/convert to radials as I go along.

With the radials, there is far more to choose from, they are much cheaper & they are typically better rated than most of what is left for axials.

The trick is figuring how to physically adapt radials to the designs since we are so accustomed to axial layouts.

[kagliostro]

I agree, only remember I was talking of a single cap (the 8uF one) the other values are on the Multi Cap Can

Using the cap that was linked by Sluckey there is no need to add a further cap to the Multi Cap Can because a 10uF (good for V1) is present on that Multi Cap

I only suggest to give a try to the Multi Cap that is now on the amp to save money

Franco

[Platefire]

I have a question about something else on the amp. The 6V6 Cathode resistance on this amp is taken from the OT as shown on the schematic at 32 ohms.Schematic is attached. My intentions were to disconnect this black lead from the 6V6 pin# 8 and cap it off and install a 5F2-A value cathode resistor and bypass cap that is 25uf/50 ecap and 470/1W R. This appears to not be a problem to me. Are there any negative connotations that I don't see?

[sluckey]

I have a question about something else on the amp. The 6V6 Cathode resistance on this amp is taken from the OT as shown on the schematic at 32 ohms.Schematic is attached. My intentions were to disconnect this black lead from the 6V6 pin# 8 and cap it off and install a 5F2-A value cathode resistor and bypass cap that is 25uf/50 ecap and 470/1W R. This appears to not be a problem to me. Are there any negative connotations that I don't see?

That black wire was an old technology way to bias the cathode and also provide negative feedback. Just disconnect the black wire and use the Fender cathode components. You'll also be using the Fender NFB circuit.
 

[Platefire]

Thanks sluckey!

[Platefire]

So I finally got around to doing the demo and got parts ordered. So some progress. It is going to be a 5F2-A with a 6X4 Rectifier and the filter  be 40/20/20/10 instead of what shown on the attached schematic. Also it will have
just one input with 33K grid stopper with the normal fender input setup wiring

[sluckey]

Looks like fun.

[Platefire]

Well??? I've tore down a lot of old P to P amps for a rebuild before but I think this one had the toughest mechanical connections I've ever tried to unravel. I ended up tearing up some of the terminals on the 9 pin socket even though I was being careful as I could. I use to think it was cool to make super strong mechanical connections but now they are done with the idea of a manageable future tear down in mind
 

[kagliostro]

... just one input with 33K grid stopper ...

if I can suggest, use this input

I mean from the input jack to pin #2 of V1
(not the whole V1 arrangement)

Franco

[sluckey]

Sometimes it's impossible to not break some delicate tube pins. I would have tackled this tear down differently. First remove the transformers just to get them out of the way. Then use wire cutters to clip all components out of the chassis, leaving about 1/2" lead length on the sockets. Do the same with any wiring. This should open up the chassis to give better access to the sockets. Then begin surgically removing the lead stubs from the sockets. Using simple desoldering aid tools such as these will give you a better chance of salvaging those sockets.

[shooter]

use wire cutters to clip all components out of the chassis, leaving about 1/2" lead length on the sockets.

In my early geek years I found an ad in the back of an electronics magazine "100 pounds of surplus electronics $9.95.." gave Mom the cash, Dad rolled his eyes, 4 months later this beat up 3 cubic ft box gets dumped in the driveway, just in time for winter!  by spring I was quite good at desoldering, unraveling leads n swearing as i pulled clipped leads I stepped on, sticking out of my toes!! 

[Platefire]

sluckey- I've got solder suckers, de-solder braid, the RS picks/punches and my patients/endurance is pretty up to par.  I'm telling you the person who installed these components intended for them to stay, with the heavy leads, double wraps, great gobs of solder plus as you say--- limited space---ate my lunch.As C.E. would say but we will adapt, adjust and move on>>>>


shooter----when I first started amping I practiced soldering and de-soldering on old junked components before building my first Champ Kit. Did a lot of reading on the subject also. I did soldering on guitars before I got into amps but I had no idea what I was doing. That was before the internet and I
just did it. Somehow it worked.

[PRR]

If you have obsessive lug-wrap, you may have to solder-suck then use small sharp snips to take it off a half a wrap at a time.

And yes the cheaper mini-sockets may not stand the abuse.

[Platefire]

I have repaired 9 pin sockets before by removing damage terms off one socket, robbing a good terms or two off another junked socket and installing them and made it work. It's just when you do that the terms or not as secure and kind of iffy. So I think on this I would go with a new socket:>). I sure don't want to have to tear it down again after I get it all wired up

[Platefire]

Got a question about shielded cable. I have used shielded cable on occasions but not very often. It seems I've been able to beat the hum and noise usually without it. On this amp however, because its so small and harder to put space in between wire runs and components, I'm planning to put shielded on all the pre amp runs from input to V1a, from V1a to volume/tone control and from Vol/tone back to v1b.

My question is this overkill? I don't want kill my highs. This is a 5f2a circuit, it may have plenty of highs regardless!

The three runs to pre amp I just discussed accesses V1 through the same hold in chassis, so I was thinking since the first two runs will be shielded, it would be ok to use regular hookup wire from tone stack to V1b?
So those of you who have a lot of experience with shielded cable, please chime in

[sluckey]

How long are these 3 wires?

[Platefire]

Approximately! input to V1a 6", V1a to Vol 8" and Vol to V1b 9"

[sluckey]

I would use shielded cable.

[Platefire]

OK Thanks. I think Doug has some pictures how to properly cut and wire the ground in on shilded. I'll have a look

[sluckey]

These are the two methods I use. Both are good solid connections. Which to use? Depends on the circumstances and sometimes just my mood. Be careful when soldering the pigtail to the shield. It's very easy to melt the insulation on the center conductor. Not a problem if you use teflon insulation such as RG-316.

[Platefire]

The black cable is RG174. The brown cable is some shielded that I got out of a Hammond PR-40 Tone cabinet. Thinking of using it because it seems a little more heavy duty

[Platefire]

Dumb simple question here. I know that sometimes the fuse and power switch is sometimes put on one lead of the PT primary's and the other primary lead goes directly to the power cord connection.  In just about all cases where this exist, the fuse is first and the switch is second in line.

My question is, would it matter it the switch was first and the fuse second or is there a good reason the fuse is always first?

[shooter]

in my world it doesn't, but but it's a happy place without many rules    in the world of Codes n such, fuse comes before switch IIRC.

[PRR]

What is before and after?

You can be shocked THROUGH the transformer winding. You can probably start a fire too.

If you were serious, you would double-fuse and double-pole switch. A DP switch is required in some countries.

[Platefire]

Yeah, but PRR this project is a very tight limited space project as you know on the VM 8810. So I'm not looking to add more than necessary. It's getting a fuse, more than it had before

[shooter]

I see the worse case as switch shorted (when you think it's off), it's NOT
that'll bite hard, and easy "cheat" wire the pilot light between switch n fuse, that way the box is telling you "I'm live"
if you ignore the box, well that's on you.


When I start up my bread oven, the label will read "This product has NOT been approved or inspected by any governmental agencies, therefore you must assume the bread will kill you...IF however.... 
maybe something like that zip tied to the power cord