Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on October 23, 2023, 12:57:00 pm
-
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.
-
Those look like pretty modern Xicon power resistors in the pic.
Part of someone's earlier re-configuration..?
-
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
-
Ciao Platefire
.... but---on the actual amp there are two 500 Ohm/10W resistors between those red leads and the rectifier.
(https://i.imgur.com/5W7Blb7.jpg)
Source
https://el34world.com/Forum/index.php?topic=20321.0 (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
(https://i.imgur.com/xZXENl4.jpg)
On this link you can read explanations about
http://www.valvewizard.co.uk/bridge.html (http://www.valvewizard.co.uk/bridge.html)
Franco
-
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. :icon_biggrin:
-
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 :dontknow:
-
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
-
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
-
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.
-
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}
-
Sim of intermediate value:
PSUD3? How/where does one get that version? I don't see it on Duncan's site.
Thanks
-
> 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.
-
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:
-
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 :think1:
-
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 (https://el34world.com/Forum/index.php?topic=14464.0)
(https://el34world.com/Forum/index.php?action=dlattach;topic=14464.0;attach=32442;image)
Franco
-
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
-
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 (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
-
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?
-
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
-
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
-
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
-
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
(https://i.imgur.com/J1noGIK.jpg)
For the 8uF you can use a small radial
Franco
-
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
-
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
-
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.
-
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
-
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?
-
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.
-
Thanks sluckey! :thumbsup:
-
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
-
Looks like fun.
-
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
-
... 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
-
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 (https://www.amazon.com/Sainwora-Soldering-Accessories-Desoldering-Tweezers/dp/B08FH6Y98Y/) such as these will give you a better chance of salvaging those sockets.
-
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!! :icon_biggrin:
-
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.
-
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.
-
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
-
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
-
How long are these 3 wires?
-
Approximately! input to V1a 6", V1a to Vol 8" and Vol to V1b 9"
-
I would use shielded cable.
-
OK Thanks. I think Doug has some pictures how to properly cut and wire the ground in on shilded. I'll have a look
-
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.
-
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
-
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?
-
in my world it doesn't, but but it's a happy place without many rules :icon_biggrin: in the world of Codes n such, fuse comes before switch IIRC.
-
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.
-
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 :icon_biggrin:
-
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.... :icon_biggrin:
maybe something like that zip tied to the power cord
-
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!
-
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??
-
Close enough.
-
Thanks sluckey!
-
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?
-
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
-
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.
-
Good! Getting close to start building it back, so checking everything out I have questions about first. Thanks!
-
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.
-
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
-
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
-
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.
-
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
-
....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.
-
Well PRR it's good to know I'm not the only one that gets one tread mixed up with another :laugh:
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 :sleepy2:
-
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 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.
-
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
-
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
-
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
-
OK, Shooter. Thanks! Does that happen to be that 1482 variation you built a while back?
-
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"
-
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.
-
.... Merlin's suggested modifications to limit bass ....
Don't remember were, can you remember it to me ?
Thanks
Franco
-
.... 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
-
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
-
Now that's the kind of chassis that I'd just throw away. Harvest the transformers and build in a fresh proper chassis. :icon_biggrin:
-
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 :icon_biggrin:
-
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!
-
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:>)
-
it should be fine. shielding works by "offering" noise a better "short" to ground than the center conductor path through it's travels
-
Huge gaps have little effect. Hammond was not so dumb.
-
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!
-
If you are having problems with solder running down into the socket pins, you may be
a red using too much solder. :icon_biggrin:
-
:laugh:
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!
-
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??
-
Weller S 200 N gun
:laugh:
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
-
> 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".
-
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
-
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
-
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
-
> 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.
-
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.
-
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.
-
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
-
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
-
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?
-
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 :icon_biggrin:
-
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:>)
-
"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.
-
Hay Thanks PRR! I guess I was thinking the melting point was a lot lower. My new station goes from ranges from 0 to about 850 F. Most of my parts are new, so I hadn't tried to clean them because they were new, but maybe I should?
So I never really thought about soldering temp much before because I had no way to gauge it, but now I'm trying to catch up:>)
BTW-What type and size solder do you use for general amp repair?
-
> What type and size solder
I bought pounds of 0.031" 60:40 tin:lead fluxcore in my career, RadioShack and Kester, and still have much of it.
I only had an iron with a knob on it once, and it didn't work good (it was a period with a real draft over my workbench and all my irons acted sick). The 45W dumb iron has a temperature on it but too late to look tonight. I do know it glows dim-red in dark, and easily lit cigarettes on the side.
-
because they were new, but maybe I should?
most new stuff will have a sheen of machine oil from the part making process
I bought pounds of 0.031" 60:40 tin:lead fluxcore in my career
yep, I use mostly Kester now. working the career i'd order the good stuff 63:37 fluxcore 1 pound in the lunch pail-toolbox, couple 1 pounder's on the repair/work bench, 1 pound at home. still got about 1/2lb roll of the 30GA for IC work.
-
OK Thanks! I seen this at Harbor Freight when I bought my Station but I wasn't sure then but looks like it aligns with what both of you are using. I'll get it when the wife and I go shopping next week
https://www.harborfreight.com/home/hobby-tools/soldering-electrical/08mm-electrical-solder-58000.html
-
OK Thanks! I seen this at Harbor Freight when I bought my Station but I wasn't sure then but looks like it aligns with what both of you are using. I'll get it when the wife and I go shopping next week
https://www.harborfreight.com/home/hobby-tools/soldering-electrical/08mm-electrical-solder-58000.html
It's the correct alloy (SN63), and definitely cheap enougj, but I see no mention if it's cored or what with.
You need a rosin flux core for electronic work.
-
Thanks,, your right. I don't see where it designates Rosin core anywhere
Looks like this link is more in order, Kester as Shooter recommend
https://www.ebay.com/itm/132402724455
-
If you can sneak a sample of that HF solder, melt a bit with whatever's handy - an iron, match or lighter. If it produces smoke, then it's rosin core.
Btw, $20/lb. is dirt cheap these days for SN63 electronics solder.
ETA: That listing you posted wants $11/oz. of solder.. $176/lb. ouch!
Check item #282828369148 - $40/lb and its the correct SN63 / #44 FLUX / 0.031" .material for general electronic work. Same roll that's on my bench.
Btw, solder flux does theoretically expire.. some faster than others, apparently. But I have some 0.020" stuff from the 90s that still works just fine.. fwiw. Point is, don't worry if it's a few years out of date. Also, try to avoid "No Clean" fluxes unless you've worked with the specific material before, and are happy with its performance.
-
> Btw, $20/lb. is dirt cheap
That's a half-pound spool. $40/lb
-
> Btw, $20/lb. is dirt cheap
That's a half-pound spool. $40/lb
Ahhh.. good catch, 8oz. spool is right!
Well heck, just get the Kester product then.. it's guaranteed to be an excellent material. Same goes for Alphacore / Alpha Metals SN63, with a P2 flux. (Alpha P1 flux is their no-clean, same caveats apply).
-
OK, good! It's settled, guess I'll go with the Kester. I think it said free shipping.
One more thing, what do ya'll use to clean your tip in between solder joints. On my old Weller gun, I used an old damp wash rag. On my new station, it uses a small square sponge that you dampen but seems I'm fighting to get it really clean.I also noticed that on you tube using what looks like a wad of metal material. Sometimes a small bead of solder clings to the tip and it hard to get off. Whats best?
-
OK, good! It's settled, guess I'll go with the Kester. I think it said free shipping.
One more thing, what do ya'll use to clean your tip in between solder joints.
An old kitchen sponge with an oblong hole cut through the middle. Keep it damp with water, distilled is ideal but straight from the tap is fine too, unless you have very hard water.
-
what do ya'll use to clean your tip in between solder joints.
I like the rear pocket from '70s era Levi's, wet, "fold" like a taco shell, lay tip between folds, pinch/wipe....quickly :icon_biggrin:
-
Outch! :sad2: I wasn't quick enough
-
Btw, If you use SN62 or SN63, always keep some SN60 around to ease reworking joints, especially on P2P wiring.
Solder-suck the SN62/63 as thoroughly as possible, then reflow with SN60. Then you can rework the joint with much greater ease than if it were still made of SN62/63.
-
Wimwalther
You lost me with those Acronyms. Don't know what you are referring to?
-
nope, but;
https://www.pcbdirectory.com/community/what-is-eutectic-solder
I called it "the perfect solder" back in my career days, mighta been just mojo like guitarist have, but I went with spending the Boss's money:)
-
> You lost me with those Acronyms. SN62 or SN63, SN60
"SN" is Sn (from Latin stannum). You say "Tin". 62 and 63 are probably percents, but which is more Eutectic is like deciding between 3.2 and 3.3 beer. Sn60 makes radio-solder with a mild pasty phase where you can push it around. (For sculpted plumbing {and cheepness} you want a lot less Sn and plenty of Pb (Lead).)
-
OK, Thanks for the explanation PRR. For what work we do I think you would want the solder to set up fast before something in the joint moves and degrades the joint
-
OK, Thanks for the explanation PRR. For what work we do I think you would want the solder to set up fast before something in the joint moves and degrades the joint
Right. You want a eutectic solder, that is, one with a very narrow (temperature wise) plastic phase. Eutectic solders go from liquid to solid almost instantaneously, with little plasticity in between. SN62 & SN63 fall into this category.
Traditional tin/lead solders alloys are designated by their tin content. SN63 is 63% tin, 37% lead. SN62 is 62% tin, 2% silver and 36% lead. This latter material is known as "silver bearing solder" and is used when soldering to precious metal plating - silver, gold, rhodium, platinum, etc. - as it prevents electrolytic leeching of these metals into the joint.
The main issue with eutectic solders is that they are much more difficult to rework compared to the common SN60/Pb40 material. This is why I suggest sucking-out & reflowing SN62/63 joints with SN60 to ease reworking.
-
....want the solder to set up fast before something in the joint moves and degrades the joint
Or set-up the joint so it won't move even without solder?
60:40 was good enough for Philco and Fender, is good enuff for me. Especially because I study my work and have been embarrassed by 'cold' joints, don't like that.
-
24 years ago when I first started soldering guitar amps I overworked mechanical Joints where if you had to desolder and undo the joint, it was a real headache. I've now have been trying to hit a happy balance of mechanical Joints of just sound enough to make a solid solder joint and not a nightmare to tare down
-
Got a question about installing the new CE Manufacturing Can Cap. All the previous can caps I've installed has been the clamp and bolt on like the JJ type. This one has the 4 metal tabs that fit through the 4 slots.
The old one I took off was a CE and it was installed with the tabs bent down to the chassis and soldered to chassis.Is that a proper way to do it or are they supposed to be twisted to hold them in? Or is one way is good as the other?
-
In the good ole days those cans were called twist lock cans. Guess why? Even then at least one twisted tab was soldered. Fender just bent the tabs over and soldered two tabs to chassis in the Princeton Reverbs. Either way is a good electrical connection. I prefer the twist lock/solder method for a good mechanical and electrical connection.
-
Got Ya! Thanks Sluckey :thumbsup:
-
Progress! Got circuit wired in. Last chore is I need to wire in AC power, power cord, fuse holder, power switch and pilot light. Not a thing of beauty being rat nest P to P but here is a few pixs
-
Finished installing AC power hookup today and did initial fire up. Was bamboozled for a while because I was getting crazy voltage readings before I put the pre and power amp tubes in. Turns out the PT wasn't grounded. Once I took care of that the voltage reading straightened out. This is the voltage readings in general:
To PT: 371 VDC
To 6V6 Plate: 369 VDC
To 6V6 Screen 351 VDC
to 12AX7 Plates 201.7/204.2 VDC
Problem 1-At first fire up I noticed an immediate high pitched squeal. First thing I thought off was NFB, so I disconnected the NFB and the squeal quit. I'm not sure whats going on here. I know with a PP amp, you swap the plate wires from OT.On a single ended don't recall having this problem before? I changed the NFB resistor from 22K to 33K. I think because of going from 4 ohm OT to 8 ohm. I did try the NFB on the neg speaker term but didn't seem to have any effect. What do I need to do to get a working NFB loop?
Problem 2-At low volume the amp is relatively quiet but as you turn the volume up hum comes in and gets louder as you turn the volume up. I have chop sticked it a little bit but haven't really improved the situation. If the hum get louder as you turn it up, don't that mean the problem is in the power amp section???
The good news is the amp is really peppy, touch sensitive, has great tone and gets pretty loud. The tone control works really good. It comes into break up a little too quick. It really needs a good NFB loop to calm it down and give it more clean headroom. The break up sounds so good harmonically, you almost want to not complain about it. If I could get rid of the hum and get more headroom, it would be wonderful.Any help appreciated, thanks! Platefire
-
high pitched squeal
what's the bias vdc on the 6V6?
-
Problem 1... There are probably only two wires on the OT primary, typically red and blue. Swap those two wires.
Problem 2... Probably layout. Don't I see the volume and tone controls located right above the PT? That's not ideal.
Where's the schematic???
-
Thanks shooter and sluckey for your replys
shooter as this is cathode biased, not sure what your referring to vdc as there is no negative voltage bias circuit???
Problem 1-Got Ya! but---access is so closed in it makes it almost impossible to get access to 6V6 terms plus the blue plate lead is just a nub being the socket is so close to the OT---I would have to splice onto it for it to reach the can caps. I know making excuses don't get the job done but it's almost a sure thing that I would tear something else up trying but I'll continue looking to make a way. See picture of access below.
Problem 2-Yelp, only 1/2" clearance between pots and transformer---see picture. Another thing I did that might be causing hum problems where I grounded the PT. Closest available ground was an empty term on a terminal strip between the two PT secondary red leads that feed the two 500 Ohm/10W resistors that go to the rectifier. I'm thinking this might be a potential source of hum---see picture. Maybe I need to relocate this ground
Schematic: I plan to make a real schematic latter but for right now me and
Leo been using his marked up one
-
not sure what your referring to vdc
cathode VDC divided by cathode resistor IS bias voltage.
had an SE once that screamed like a PP with it leads backwards, the bias was about 2X hotter than it shoulda been.
it's easier than hunting down inductive coupled signals in a rats nest :icon_biggrin:
-
shooter----Cathode DC 20.92 divided by 470=.0445
-
Problem 1-Got Ya! but---access is so closed in it makes it almost impossible to get access to 6V6 terms plus the blue plate lead is just a nub being the socket is so close to the OT---I would have to splice onto it for it to reach the can caps. I know making excuses don't get the job done but it's almost a sure thing that I would tear something else up trying but I'll continue looking to make a way. See picture of access below.
Well then, swap the two OT secondary leads at the speaker jack. Does the same thing.
Problem 2-Yelp, only 1/2" clearance between pots and transformer---see picture. Another thing I did that might be causing hum problems where I grounded the PT. Closest available ground was an empty term on a terminal strip between the two PT secondary red leads that feed the two 500 Ohm/10W resistors that go to the rectifier. I'm thinking this might be a potential source of hum---see picture. Maybe I need to relocate this ground
Ain't no cure for the summertime blues! Throw that crappy chassis away and rebuild in a more guitar friendly chassis. Even a shoe box is better than what you have now. :BangHead:
-
If my figures are right this amp is biased at 16.57 watts dissipation! That's about 5 watt over max
Cathode resistor measures: 476.6 OhmsVoltage drop across cathode R: 20.98 VDC
PD=(PCma)(PVv) = 20.98 Ma divided by 476.6Ohm=.044 A or 44 Ma
(PV)(.044A)=(376.7) (.044)=16.57 Watts Dis
If this is right, I am way to Hot?
-
EDIT ya beat me!!!
Cathode DC 20.92 divided by 470=.0445
To 6V6 Plate: 369 VDC
369-21=349
349*.0445=15.5W i'm happy now :laugh:
so swap the speaker wires
is the speaker jack grounded to the chassis
-
Wow Thanks! You guys did it :happy1:
I swapped the speaker wires from OT to speaker jack and hooked up the NFB wire and Wow---just what was needed. It's Fender clean up to about 12:00 on the volume. Then from there on the breakup starts kicking in with a lot of added volume. So IMHO is a good balance of clean and break up. The hum is not completely gone but is somewhat diminished to the point where I think it may be tolerable.
So I'm thinking if the bias is OK, we're pretty much there. I'll keep testing and see :huh:
-
eature=shared
Sound Clip of this amp
-
This is my first draft of the schematic with the 5F2-A married to VM 8810 transformers and rectifier. You see any mistakes,please let me know
-
I have an 8810 as well. I kept part of the wiring and removed other parts and built a 5f1 circuit. Very tight space. I did have to drill out some bigger holes and added a switch and fuse and didn't use the cap can.
I use just one input jack with a 33k resistor but there is another hole to add another input jack with 2 68k resistors for hi/low inputs.
My question is regarding the AC cord. The original had a on/off switch connected to the volume control on top of chassis. I put a fuse and power switch at the top of chassis.
Technically, if a person does not unplug the amp if changing a tube there would be AC present nearby. If I sold or gave away the amp how it's presently wired is this something I need to be concerned about?
Or the new owner concerned about?
-
Technically, if a person does not unplug the amp if changing a tube there would be AC present nearby. If I sold or gave away the amp how it's presently wired is this something I need to be concerned about?
Or the new owner concerned about?
Yes! At a minimum, put a caution sticker on the PT that basically states "SHOCK HAZARD! Unplug power cord before changing tubes."Even better to use some heat shrink tubing to insulate that fuse holder.
Or, and everyone is gonna love this... Swap the power cord white and black wires. No more shock hazard. :l2:
-
I like the sticker with shock hazard which I wouldn't have thought to do.
I will do the heat shrink, I wasn't sure about what to use.
Maybe on the neutral.
-
I like the sticker with shock hazard which I wouldn't have thought to do.
I will do the heat shrink, I wasn't sure about what to use.
Maybe on the neutral.
Heat shrink is easy. Just get some 1" diameter tubing from electrical parts store or even Lowe's, Home Depot, etc...
-
Just passing through to see what's going on here. Looks like all is well :thumbsup: