Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wsscott on March 12, 2021, 10:29:01 am
-
I'm hooked on Magnatone's sound, and after building the Sluckey M-10A without the power supply so its just Vibrato and Reverb, I thought I would like to try building just a Magnatone Amp with Tremolo. But I wanted to base it on the new Magnatones that are being built. I really like the sound of the Twilighter model with the 6V6's and the 12" speaker that I've only heard on the Magnatone USA website, and maybe some YouTube channels. But since I have a Vibrato/Reverb effects unit, I wanted to simplify the circuit of the Twilighter, leave those parts out, but keep the Tremolo circuit in. I also was thinking about changing the grounding system so that its what I think is called a "floating ground", which would hopefully eliminate any ground loop hum when another AC powered effects unit, like my Vibrato/Reverb, is plugged in and used. Maybe that's not a good idea. I requested Magnatone for a copy of their schematic for the Twilighter and they sent it to me with no restriction on sharing. I told them what I was thinking of doing. So at this stage, its still in the concept stage, but I really enjoyed my last scratch, modified, project so much and got such great help from the folks on this forum, I thought I would throw this out there, and see what your thoughts are. Your input is greatly appreciated. Thanks.
-
Can we see the schematic?
-
Sure, here you go. Magnatone Tech Support said it was fine for me to share.
-
The amp is a Fender AB763 Deluxe Reverb. Preamp and reverb are identical. PI and PA are 95% identical. The vibrato modulator is sandwiched between the preamp and PI. The oscillator signal is applied to the 6V6 grids when in Tremolo mode, very similar to the 6G12 trem that Hoffman uses.
-
Sluckey-Any thoughts on my comments above on what I'd like to do with it. In addition, I'd probably also want to add a Mid-Tone Control and a Master Volume and also maybe a line-input and also line-output.
-
Build Hoffman's single channel AB763.
-
I'll take a look at it. Thanks.
-
Print the two schematics and put them side by side. You'll see what I mean.
-
I printed out both schematics and compared them as Sluckey suggested, and yes they probably are "95%" the same. I don't want the vibrato or the reverb from either build, but I do want the tremolo from the Magnatone. As I said before, I have a vibrato/reverb Mag mod that I built, and that's what I would use. So I would take those out of both circuits. The Deluxe has the Mid which I would want to add to the Mag circuit. The Mag circuit has the line-out which I want. Neither have a Master Volume which I'd like to add. That might be about it. Is there a DIYLC for the AB763? I didn't see one on Hoffman's site, but I did see one on Rob Robinette's site. So I could start from scratch doing a layout with DIYLC, or find a close match and then start modifying from there. I know you probably think I'm spending a lot of unnecessary time not just going straight with the Hoffman/Sluckey design/layout, but I don't have anything else to do and I know I will learn something in the process! Thoughts?
-
You'll be better off in the long run by drawing up a schematic. Either by hand or via whichever program you use.
A schematic really helps the forum help you better.
Once you have a stable schematic, the layout follows easily (on a fresh build), and you can have Doug drill your board on his CNC machine.
-
Here's your stripped down schematic. Have fun.
-
Sluckey- Gee Thanks again--How did you do that? Is there a program you have that can do it quickly? Wow!!
-
Acrobat Pro and PaintShop Pro.
-
Sluckey--I really appreciate it. You know you haven't heard the last of me! HaHa! Best.
-
Now I'm trying to find the dimensions for the Twilighter chassis so I can get an idea on the size of the turret board(s) and ultimately cabinet. I like the way the front of the amp is just grill cloth and no controls, and its easier to remove the chassis than from a Fender Blackface style. I dealt with that Princeton Reverb and it was always a little "sticky" getting it out of the cabinet.
-
From: https://www.magnatoneusa.com/twilighter
Twilighter (not the stereo version): 22.5" x 10" x 19.5"
-
I think that’s the dimensions for the cabinet. I need the chassis dimensions for the circuit board and other guts.
-
I could easily fit that MT Lite schematic into a 12 x 6.5 x 2.5 chassis, or even Hoffman's pre-punched Stout chassis. A 17 x 6.5 x 2.5 chassis would look spacious inside.
Look at my Dual Lite, or Plexi 6V6, or L'il Maggie, or Phoenix. All built in a 12 x 6.5 x 2.5 chassis. In fact, I would use the Phoenix layout because 90% of the layout would be identical to the MT Lite. Simply replace the Ampeg preamp with the MT Lite tremolo.
http://sluckeyamps.com/dual_lite/dual_lite.htm
http://sluckeyamps.com/6v6plexi/6v6plexi.htm
http://sluckeyamps.com/lil_maggie/lil_maggie.htm
http://sluckeyamps.com/phoenix/phoenix.htm
-
Thanks, I'll do that. While I've got you I have 2 questions about the Twilighter schematic symbols, if you could help.
First, in the tremolo section, near the Speed pot, to its right, is the label "LFO". What is that for? Is it a component or just identifying the part of the circuit?
Second, in various places in the schematic there are 2 *'s shown like this "**". There are several shown in the tremolo and other parts of the schematic. I can't find a reference, so I'm curious as to what they are indicating.
Thanks.
-
LFO = Low Frequency Oscillator. That's the name for the entire V5B oscillator circuit. LFO is probably stamped on the circuit board next to a test point (P$1).
The note (** DO NOT INSERT) can be seen on the left side of the schematic near the LFO circuit. It's not important so I'm not even gonna guess.
-
Thanks. I should have seen that ** note.
If I wanted to reduce the power output for some reason, for ex. a friend likes the 14 Watts of his Princeton Reverb and wonders if 22 Watts is too much, could I just replace the 6V6's in this with a cooler tube, and then adjust the bias, or would there be more to it?
-
a friend likes the 14 Watts of his Princeton Reverb and wonders if 22 Watts is too much, could I just replace the 6V6's in this with a cooler tube, and then adjust the bias, or would there be more to it?
Then build a Princeton Reverb. or a 6G2 Brown Princeton with the 125P1A power transformer. lower B+, still two 6V6's. heralded as perhaps the best princeton Fender made. Probably among the top five of all Fenders (if there was/is such a ranking).
-
add a self biased/fixed bias setup, although the "perceived" difference probably isn't worth the effort.
I'd build it as is, then;
bias cool
add less efficient speaker
use pedals
turn MV down
walla, ~~~ 15W
-
With a Master Volume added to the circuit, that might be a solution?
The Line-Out shown on the Twilghter Schematic and as on the modified MT-Lite that Sluckey did for me shows a cap, and I'm trying to confirm what it is. Is it a 1 ohm 630V polypropelene capacitor, or is it something else?
As to the filter caps, its showing a 40 and then 20 20 20 8. I was thinking of using a Cap Can for the 40 20 20 20 and then a separate 8 ohm. How does that sound?
-
Also, to clarify, in the Tremolo section is that cap identified as " C_220_10V_Radial" a 220 ohm 10 V radial electrolytic?
Does it have to be radial?
-
Is it a 1 ohm 630V polypropelene capacitor, or is it something else?
in the Tremolo section is that cap identified as " C_220_10V_Radial" a 220 ohm 10 V radial electrolytic?
Caps are not measured in ohms!
The line out cap is 1µF @ 63V and poly is stated rather than an electrolytic. The trem cap is a 220µF @ 10V electrolytic. Radial specified because it is small.
-
I know, it must be slipping! Thanks for the clarification.
-
Sluckey, for this build, as to the filter caps, its showing a 40 and then 20 20 20 8. I was thinking of using a Cap Can for the 40 20 20 20 and then a separate 8 ohm. How does that sound?
-
separate 8 ohm
8uF not ohm
should be fine
-
Right. Thanks.
-
Should be easier to look at...
EDIT... Preliminary drawing deleted. Final drawing posted in reply #50.
-
Boy, that's great. Much better. Thanks again.
-
Sluckey, to add a Master Volume pot, would I just wire it between the coupling cap going to the PI Grid pin2 and GND, or is there another/better way to add it?
-
Just replace that 220K/270K voltage divider with a 500K-A pot.
-
Thanks. I also didn't know if it should come before or after the PI.
-
Magnatone didn't think the MV was worthwhile. Neither do I.
-
One of the reasons I was thinking the Master Volume might be useful is that a friend who has a small recording studio might be using the amp some, and you could get the break-up or distortion, etc. but not at high volumes. He currently uses a Princeton Reverb which has been working fine for his needs. Otherwise, I wouldn't be adding it. Would the Master Volume be effective for that purpose?
-
since it's a studio, he can use 12 amps, mike them, tweak them, blend/mix them.
build your amp as designed. If a MV becomes a "must", you can shoe-horn it inside an hour after the fact
-
Makes sense. I just need to allow for a location for the pot.
-
Sluckey-I'm going to start working on the DIYLC layout tomorrow. A couple of questions.
Is there one DIYLC already at Hoffman, or elsewhere that you know of, that I could use to get a quick start for this layout?
And, the schematics are designed from left to right, but the layout is designed from right to left. Am I missing something. That's the way I did the layout for the Mag10A/VibroVerb mod, and sometimes it was a bit confusing, but I wanted to make sure I wasn't missing a better way to do the layout.
Thanks.
-
I know nothing about DIYLC.
My Visio layouts are from right to left, just like my amps look when flipped over. I'm pretty sure Hoffman did his Visio layouts from left to right.
Get your mind straight on this. Otherwise you risk designing a mirror image board. Usually shows up after you drill the chassis. :laugh:
-
Yep, I know what you mean.
-
Sluckey, I'm starting to work on the Layout today, I wanted to know where in the layout you think I should put the components for the Tremolo Oscillator. I see where it ties into the circuit at the "MOD" point. But I'm trying to figure out where so the circuit is easiest to follow. Thanks.
-
Here's a preliminary drawing. Still need to proofread a couple times...
EDIT... Preliminary drawing deleted. Final drawing posted in reply #50.
-
Wow Sluckey-I really didn't expect you to do all this work on this for me! Its really great. I certainly appreciate your time. Thanks. I'm sure there aren't any mistakes, but I've learned from you and others to review numerous times before thinking its done.
-
Haha. I've already fixed two component values. A lot of that layout came from my AB763 Lite. Mainly had to just shoehorn the trem circuit between the PI and PA. Very easy to do a drill guide template once I'm confident there are no more errors.
Are you really interested to build this project, either with my layout or your own? You know, everything that was Magnatone has been stripped away and all that's left is a single channel AB763 with bias vary tremolo. I was really surprised to see that Magnatone simply copied a Deluxe Reverb and added a single stage vibrato circuit. The original Magnatone amps had very unique circuits that were well thought out.
-
Well I've only heard the sound on Magnatone's site and YouTube, but it sounds really nice to me. So that's why I'm interested in it. Have you compared this new circuit with their old Twilighter circuit? I was just curious how much of a difference there is. If there's a significant difference that you think might make this one better, I'm certainly open to other changes. Anyway, I'm still good to go with this one unless you have some suggestions for changes to it, and I'll probably use your layout. I'm sure it would be much better than mine. But I may continue with mine just to do it. Thanks again.
-
I did put a couple of small changes in it. You mentioned a MID control so I added that, but used a 250K pot which operates as a mid control from 0 to 5 but acts as a raw control from 6 to 10. And I used a Volume pot with a push/pull switch to add a bright switch. And I did not include a STBY switch. None of this affects the board layout.
I'll proof the drawings again tomorrow morning over coffee and then post the final sometime tomorrow. It will include an updated schematic, layout, and drill guide.
You may want to visit this website. The creator is a current member here...
http://www.magnatoneamps.com/
-
Thanks! Great, I really appreciate it.
-
Oh yes, I've visited that site many times over the last year or more! Good stuff.
-
I think the schematic/layout file is correct. Destroy all previous versions. LMK if you spot any errors or have questions.
-
Thanks so much. Will do.
One question, I think you said you put in the brightness option, so is that some kind of a separate SPST toggle switch of some sort, or is it a push/pull 1 M-A pot of some sort?
I was thinking I might add to the length of the board a bit and install some extra turrets in case I wanted to come back later and add a reverb. I'd also include an extra tube socket, and related mods to the chassis. If I did want to add a reverb, would you add the tube reverb circuit from the M10A mod that I did, or the one from this Mag Twilighter circuit or the one from the Hoffman Deluxe Reverb circuit?
-
One question, I think you said you put in the brightness option, so is that some kind of a separate SPST toggle switch of some sort, or is it a push/pull 1 M-A pot of some sort?
Look at the schematic and layout.
I was thinking I might add to the length of the board a bit and install some extra turrets in case I wanted to come back later and add a reverb. I'd also include an extra tube socket, and related mods to the chassis. If I did want to add a reverb, would you add the tube reverb circuit from the M10A mod that I did, or the one from this Mag Twilighter circuit or the one from the Hoffman Deluxe Reverb circuit?
Use the Twilighter one tube reverb and put it on the preamp end of the board.
-
Thanks.
-
Sluckey- In the Layout section, pre-amp section, it looks like you're showing .0047uf caps instead of .047uf as shown on the schematic.
The layout shows a .022uf cap connecting to the grid of V2 and I think it should be a .0022uf based on the schematic.
The 82K R coming off of V2-Pin 6 should be going to B, but appears to be going to C node or is the 10K 3W R blocking it going to C and thus its at B node?
What does "NFB" mean on the External Speaker Jack?
I don't see the .01uf 2KV Cap between the Standby at the Choke and the OT.
A minor point, but some of the tubes on the schematic are mis-labeled, ie. they still show their original designation from the original schematic: See V4 & V5 mis-labeled as "V6" and "V7"; V3 mis-labeled as "V5-A".
I may have misinterpreted the Layout, and sorry if I screwed up with these comments.
-
Sluckey- In the Layout section, pre-amp section, it looks like you're showing .0047uf caps instead of .047uf as shown on the schematic. fixed
The layout shows a .022uf cap connecting to the grid of V2 and I think it should be a .0022uf based on the schematic. fixed
The 82K R coming off of V2-Pin 6 should be going to B, but appears to be going to C node or is the 10K 3W R blocking it going to C and thus its at B node? There is no 82K coming off of V2-6. Both V2 plates correctly connect to Node B.
What does "NFB" mean on the External Speaker Jack? NFB = negative feedback
I don't see the .01uf 2KV Cap between the Standby at the Choke and the OT. fixed
A minor point, but some of the tubes on the schematic are mis-labeled, ie. they still show their original designation from the original schematic: See V4 & V5 mis-labeled as "V6" and "V7"; V3 mis-labeled as "V5-A". fixed
THX for catching those errors. LMK if you spot others. I'm uploading the fixed file into post #50. Please delete the old one.
-
Thanks
-
Yes, I meant "plates" but said "grids" and should have referred to V-2 Pin 1 not Pin 6. Sorry, sloppy of me.
-
Hey, we're getting it fine tuned. Keep on looking. :icon_biggrin:
-
Will do.
-
I've been looking for output transformers, and of course Classic Tone is out of business and they're hard to find. Also, some of the Hammonds are out of stock. I did find a Hammond 1760H available, and have attached the data sheet, and wanted to know if you think this would be a good fit. I really don't need the flexibility of 4 and 16 ohms, but its there. Any other choices you recommend?
-
Or this Hammond 1760J?
-
Sluckey-I've compared the original Twilighter circuit from Magnatone with your Visio schematic 2 times, and it matches up. I also have gone over your revised Layout 2 times and compared it to your Visio version of the schematic and they match up. So looks to me like this version is good to go.
Three more questions about components.
As to the brightness, push-pull, pot, I've looked around for the pot and I've seen comments about one particular brand being poorly built. Is there one that you would recommend? Alternatively, is there a different kind of switch, like a toggle switch I could use instead. Would it be a SPST or DPST? In fact I probably would prefer that. Recommendations?
Then as to the Bias Trim pot, I've got one on order that looks like a typical 3 terminal pot, made by Alpha, and it can be mounted to the chassis like a regular pot. Access to the adjustment screw slot would be on the outside of the chassis. Any reason not to use it? Is it really inconvenient to be adjusting it from the outside of the chassis vs. inside the chassis?
Finally, the way I'm planning on designing the chassis is to have it similar in design to the Mag. Twilighter chassis. The Layout has the tubes located on the opposite side of the turret board from the pots. In my design though, that would put the tubes way back into the cabinet near the speaker. So I would want to move them towards the front of the chassis so that they are visible and accessible from the rear of the cabinet. If I move them to the front side of the turret board in the chassis, that would work, but would it affect the wiring or circuity in any way? I assume not. I would just be running the hookup wire under the turret board to the front of the chassis instead of to the rear as is in the Layout. Any thoughts on this placement issue?
Thanks.
-
1. You can use any kind of switch. Just mount it close to the VOL pot. This is the pot/switch I use for a bright switch...
https://www.tubesandmore.com/products/potentiometer-cts-audio-1-m-spst-push-pull-solder-lug
2. I would not use a bias pot with a shaft unless you get an expensive pot with a locknut to prevent accidental turning! Hoffman sells a CTS 10kl Fender Bias Pot that is chassis mount but can be adjusted only with a screwdriver from either side of the chassis. Very handy and safe. My board was designed to use the onboard bias trimmer pot that Hoffman sells.
3. You can layout the chassis any way you want, but... My board layout was designed to work together with the tube placement and control panel. If you deviate much from my layout you probably should redesign the board too.
-
I've tweaked the board layout slightly. Much neater bias section. The updated file is in post #50. Please destroy previous versions.
This is my final tweak unless you discover any more errors.
-
Thanks again.
-
Any thoughts on the Output Transformer?
-
as long as the OT meets current requirements and basic 0 to ~~ 8khz they all work. The speaker is where you want to fret
-
Any thoughts on the Output Transformer?
At some point you mentioned 15 watts. If that's what you want then use a Princeton Reverb PT and OT. If you want 22 watts then use a Deluxe Reverb PT and OT. Since Magnatone borrowed the DR circuit from Fender, I'd just use all DR iron,,, PT, OT, and choke.
-
I don't intend to change the Layout of the turret board. It looks great and is accurate.
In order to get the tubes on the front end of the chassis, ie. opposite to where they are now, what if I just turn the turret board, and the pots, etc., 180 degrees when I install it. Then I could put the tubes on the front end along with where the pots are? Any issues with tube socket proximity to pots and input jacks?
-
what if I just turn the turret board, and the pots, etc., 180 degrees when I install it.
That put's the sensitive inputs and preamp on the PT end of the chassis. You'd be better off just to slide all the tubes to the top of the layout. Wiring will not be optimized nor as neat.
Why do you want to do this? This stripped down amp is 95% Fender and only 5% Magnatone. Had I known you wanted tubes on the control side I would have laid out the board to accommodate.
-
I meant I would just switch the entire physical layout around, not just the board. So the PT and related components would be flipped. I want to do it like this because I want the control panel to be on the backside of the amp, and not the front like the blackface Fender's etc. I want the front of the amp just to be grill cloth. So your layout is fine, I'm just trying to humor myself with the clean look I like with amps from the front.
-
Just slide all the tubes and cap can to the top. And slide the board, PT, and choke down.
-
This seems to be what you have in mind. Will you also bring all that stuff from the bottom up to the control panel?
-
Yes, that's the idea. The power and standby switches, and the Line Out and Remote Speaker jacks would be on the front panel, and the rest would be on the bottom of the chassis but up front towards the control panel. The IEC and fuse will be on the bottom of the chassis in the front with everything else.
I assumed though, that I should flip the Turret Board 180 degrees so that the connections from the board to the tube sockets would be short, and if I did that then I'd need to put the PT and the OT on the other end too in order to keep it away from the pre-amp.
-
Sluckey-does your software generate a BOM? I'm working on one in Excel, but I'm not very good with sorting the data. Don't fool with it if your software can't generate it.
-
Don't rotate the board! Just flip it over after drilling and install the turrets on what used to be the bottom. Should look like the attached. You can make this layout or something similar work. Wiring may look messy in some spots but may look better in others.
I don't do BOMs. All the small components can be found at Hoffman. AES, and Mouser. Iron and cap can from AES. No one has everything you'll need. Some of the oddball caps I found at Mouser.
-
Great. Thanks.
-
I had a question about the signal path through the Treble Boost/Bright part of the circuit.
Am I correct that the signal comes out stage 1 of the pre-amp thru the Treble pot, and then heads into the Volume pot where it splits going into that pot. The main part of the signal goes through the Volume pot at ALL times, but if the DPST switch is in the ON position, part of the signal is diverted and goes through the 120pf cap to the Output of that pot so that it mixes with the signal going to the grid of stage 2 of the 12AX7.
Am asking because I've decided to use a toggle switch, DPST I believe, rather than the push-pull pot, and want to be sure how I wire it up. Thanks.
-
The bright switch allows high frequencies to bypass the resistance of the volume pot. That's what makes it a bright switch. At full volume it is shorted out and does nothing. I used a toggle for bright on my TDR which is the same as your amp except I opted to have reverb rather than tremolo. Fender used a slide switch. I later turned to the switch/pot just for it's one hole simplicity.
(http://sluckeyamps.com/tdr/tdr_12.jpg)
-
Am I correct that a DPST switch is the proper type?
-
Am I correct that a DPST switch is the proper type?
Any switch will work. Your DPDT will work but you are only gonna use two contacts. In fact, Fender used a DPDT slide switch even though he only used two contacts. SPST is all that's needed.
-
Thanks.
-
I was watching a YouTube video today comparing a Deluxe Reverb with a Princeton Reverb, and the guy was commenting that the DR in the Vibrato Channel is extremely loud and piercing even at low levels because of the Bright Cap on the Input for that Channel. He was saying that even at low settings, like 3, which is at the volume level you would want in order to get the clean sound, its extremely loud. He was talking about ways to get the volume level down to a home or recording studio level but still maintaining the tone.
I would think that the circuit Sluckey came up with, that the issue is really moot because you can turn off the Bright effect. Thoughts?
-
and the guy was commenting that the DR in the Vibrato Channel is extremely loud and piercing even at low levels because of the Bright Cap on the Input for that Channel.
That's bullshit. The reason the DR is louder than the PR is because the PR's PI has no gain and the DR's PI has lots of gain. However, I know how to play a DR just as quietly as a PR. :icon_biggrin:
Your amp will be louder than a PR too, with or without the bright cap.
-
Yes, there are a lot of know it all on YouTube and a lot of them are full of BS. Since I've never played or even hear a DR, what do you do to get the sound you like at listening level/comfortable volumes?
-
the sound you like
that's like askin boxers or briefs :icon_biggrin:
the soldering pencil isn't even hot, you're getting ahead of yourself :laugh:
-
I would have been a good Boy Scout: Be Prepared! Haha!
Speaking of soldering, what temp do you guys normally run? I normally run at 790 degrees, and wondered if that’s too hot. Thanks.
-
what do you do to get the sound you like at listening level/comfortable volumes?
I turn the volume down.
-
Be Prepared! Haha!
Nixon signed my Eagle Scout card :laugh:
One of the better axioms for life
-
I solder @ 700F.
-
Sluckey, what’s the angle for the Mag chassis? It looks about 45 degrees. I’m waiting for parts to arrive, so I thought I would figure out the chassis dimensions and details so I can contact a cabinet source. Guitar Cabinets Direct did the cabinet for me before and did a great job, and did some minor mods that weren’t expensive. Thanks.
-
No idea.
-
I got the parts and have started installing them on the turret board. I'm using the original configuration you did Sluckey. I've decided not to "reverse" everything and just install the chassis so its like a standard Blackface.
Issue, I ordered a 220uf 25V cap that goes in the tremolo circuit coming off the cathode on V3 along with the 3.3K resistor. Your layout specifies 35V. Will the 25V that I got work without the risk of blowing, or should I order a 35V cap? Thanks.
-
Look at the original schematic.
-
Ok, it calls for a 10V, I guess you just added some extra protection, so my 25V should be fine.
-
Now, if I had just bought some 82K ohm resistors instead of the 82 ohm resistors that I in fact purchased, the turret board would be complete. I've got 68K ohms and 100K ohms resistors, but do I have an 82K ohm resistor. No. But Oh NOOOOOO, I couldn't have gotten it right the first time. So I guess that will wait until I order some tubes or something else in order to justify the shipping!
-
Ok, it calls for a 10V, I guess you just added some extra protection, so my 25V should be fine.
I couldn't find a 22u/10v radial at my main suppliers so I bumped the voltage rating up a bit. Besides, I already had a stencil for the 35V cap. :icon_biggrin:
Now, if I had just bought some 82K ohm resistors
Get a 5-pack. That's a very common resistor for Fender amps.
Could you post a pic of your board? I'm curious about the fit of that big ole .68µF. I have never held one to do my actual measurements. Just had to trust AES's numbers.
-
Attached is the turret board. The .68 cap fit perfectly. The turrets you see that aren't connected are for future use if I decide to add in the reverb. Also, because I added those turrets I had to go under-board to be able to connect the .0022uf cap to the 1M ohm resistor.
-
You used two .68uf/100v! That 100v cap will probably work for the cap that is labeled .68uf/250v. But the ***BIG*** .68uf that has no voltage listed on my layout must be at least 500V rating. Your 100v cap will quickly fail.
-
Thanks. A few more things to add to my list.
-
I checked the AES site, and their .68uf at 630V should work fine for the 500V cap, but at 0.61" it's a bit too wide for the 250V cap. I didn't see any .68uf 250V caps on their site except for one at 200V that was really expensive. I guess I could just use long leads and have it sit above the other components, unless you have another source for the cap or another idea.
Also, I haven't been able to find a 1uf, 63V cap for the Line Out. I have found a 50V and a 630V. Thoughts?
And although I won't need tubes for awhile, I thought I'd ask your opinion on brands. What about JJ tubes for this build?
-
I checked the AES site, and their .68uf at 630V should work fine for the 500V cap, but at 0.61" it's a bit too wide for the 250V cap.
I don't know where you found a 0.61 dimension. The 630v cap I would use is 1.14 x .406 x .66 (LWH) It will fit in either position. But your 100v cap should be OK for the 250v shown on my layout. The only voltage it will see is the bias voltage.
https://www.tubesandmore.com/products/capacitor-630v-polypropylene-radial-leads
Also, I haven't been able to find a 1uf, 63V cap for the Line Out. I have found a 50V and a 630V. Thoughts?
50V is fine. DO NOT USE AN ELECTROLYTIC!
http://smallbear-electronics.mybigcommerce.com/capacitor-bc-components-1-uf-63v/
And although I won't need tubes for awhile, I thought I'd ask your opinion on brands. What about JJ tubes for this build?
I have bought a lot of JJs.
-
I was looking at the Metal Film Tubular caps. Your suggestion looks a lot better. I'll be buying more than 1 of them, so is there any reason not to use a .68uf at 630V in both locations?
-
so is there any reason not to use a .68uf at 630V in both locations?
The only reason would be if it doesn't fit.
Small Bear has several options for that 1µF film cap. DO NOT USE AN ELECTROLYTIC!
-
Gotcha-
-
Does anyone have a Mojotone chassis that they can tell me the specs for the angle of the front, and its heighth. I thought it was 45 degrees and 2 1/2" but I think that's too severe of an angle for the jacks and pilot light to fit properly.
-
I got it resolved. Waiting to get chassis fabricated.
-
Status update, for anyone who is interested: Finally got the chassis design resolved, and its in the process of fabrication. Maybe I'll get it in the next week. Trying to do custom cabinet mod via email is difficult for my project, so I and the fabricator have decided to ship them the chassis when I get it and they will build the cabinet to fit. Its just a few mods to a Brownface Deluxe cabinet, but not being a cabinet builder, its hard for me to think of the changes that need to be considered. This will turn out the best way to do it for me.
-
Well the 0.68uf 650V caps and the 82K resistor came in yesterday, and I installed them this morning. Attached is a photo. I think the caps fit fine. I decided to use the BIG ones for both. So the turret board is complete and waiting for the chassis to proceed.
-
Well the Chassis came in this week, and I sent it to the cabinet fabricator to use for the cabinet design. Hopefully that will go smoothly, and maybe in a week or so I'll have it and the chassis back, so I can start wiring it up.
Sluckey, 2 questions for you. On your layout, it shows the wire for the Negative Feedback circuit as having a different color. Did you just do that to make it easier to see, or is there something special/different about this wire. Also, as to the 1 ohm bias checkpoint resistors, are they just 1/2 watt like all the others or is there something special about them also?
I've never had to cut rectangular openings in an aluminum chassis before, so I wanted to ask for some guidance on the best/easiest way to do that. I'll need cutouts for the PT and for the IEC jack. The chassis is 0.08" aluminum. I was thinking of drilling holes in each corner of the marked rectangle area to be cut out, and then use a fine tooth, 32 TPI, manual hacksaw to cut it out. Thoughts?
Thanks as always. Stephen
-
drilling holes in each corner
I use a dermal with metal cutting wheel, take your time, then finish the fine work with a flat file
-
Sluckey, 2 questions for you. On your layout, it shows the wire for the Negative Feedback circuit as having a different color. Did you just do that to make it easier to see, or is there something special/different about this wire.
Seriously? Is that really a question?
Also, as to the 1 ohm bias checkpoint resistors, are they just 1/2 watt like all the others or is there something special about them also?
Half watt is fine although I use some 1Ω/1W/1% that I got from Mouser a long time ago.
I've never had to cut rectangular openings in an aluminum chassis before, so I wanted to ask for some guidance on the best/easiest way to do that. I'll need cutouts for the PT and for the IEC jack. The chassis is 0.08" aluminum. I was thinking of drilling holes in each corner of the marked rectangle area to be cut out, and then use a fine tooth, 32 TPI, manual hacksaw to cut it out. Thoughts?
I drill corners then use Dremel with cutoff wheel and finish with files.
-
Well Sluckey, I hate to be stupid, but yes its a real question. It has a notation, "NFB" on it, plus it's color stands out. Why put the notation and the colorization on the Layout if it doesn't mean something? The schematic shows it as wired normally, nothing unusual, so that's the reason.
-
Did you also notice that the NFB wire is smaller than the other wires? Or were you saving that for a follow-up question? :icon_biggrin:
-
Actually, no I hadn't noticed that the wire is smaller until you mention it now. Thanks.
-
Old men just gotta have a little fun every now and then. :icon_biggrin:
-
Old men just gotta have a little fun every now and then. :icon_biggrin:
:laugh:
-
Well, as Paul McCartney says everybody needs to "Dance Tonight". Good song.
-
Just to let anyone who’s interested know that neither the project nor I am dead yet. The cabinet probably won’t be ready until late June, so I’m on hold until then. But since I had ankle replacement surgery last week at UVA, the cabinet build will probably fit in with my recovery schedule anyway. Hope you guys are doing well, and that you have some interesting new projects on your workbenches. Best.
-
Well, I have arisen from the dead! My cabinet just arrived this week, so now I have a chassis and a cabinet that fit together great. Now that I have the chassis back I plan to start working on the layout on the chassis next week. The last chassis I did from scratch was the Mag 10A Mod that Sluckey, Shooter and others helped me on last year. When I drilled the holes for the controls on the front they were in pretty good alignment, but although I used a punch, and a drill press, and a stepped bit, some of them wound up being slightly off the center punch. Not a big deal on that chassis because they were so close as not really noticeable, and I wasn't going to have a faceplate or something to go over it. But I'm thinking of having some kind of a faceplate this time so I really want them to be accurate. I was thinking maybe I should make a template from some fiber board or something to use as a drill guide. Any thoughts? Thanks again for your comments, interest and time. Stephen
-
make a template
:icon_biggrin:
if you have a custom overlay faceplate, that "hides" many a drift on the backing plate
-
First off. Don't start your holes with a step bit. Accurately mark with the center punch. Use a 1/16" bit to drill pilot holes. Then use a 1/8" bit to make the pilots larger. THEN switch to the step bit.
If you want to ensure the faceplate lines up with the holes in the chassis, then attach the faceplate to the chassis, mark, center punch, and drill both at the same time.
-
Yes, last time I used a center punch, then a small drill, then step drill, so probably adding another interim sized drill would help. The faceplate would be custom so I don’t have one yet. I’m going to see if a local graphics/sign shop can do something for me. Obviously to drill the holes I’ll know what the layout is, so maybe it’s something they can do now and I can drill both together as Sluckey suggests. Problem may be finding someone local who can do it. Maybe my project for this weekend is to do the layout design.
-
see if a local graphics/sign shop can do something for me.
maybe find your local fabrication shop and see what they can do to center n drill the holes :dontknow:
-
I'm thinking an all-round sign shop must have a drill. Whether they can drill better than you is a different question; my sign shop, I don't trust the young guy much but I bet the old guy can do anything he reckons he can. Or next door in the steel shop they can drill ANYthing to any spec, but they charge for tools and skill.
-
I’m making some progress!
-
Jacks on the left seem higher than jacks on the right.
-
> Jacks on the left seem higher than jacks on the right.
Less than 1/8".
Man, I'd shove you off the stage for pointing that out.
-
Haha. But it's so obvious to someone who has drilled a lot of holes. Most people would never notice. Might make getting a faceplate that fits a little challenge. Good argument for not drilling until the faceplate is made, then using the faceplate as a drill template.
-
> Jacks on the left seem higher than jacks on the right.
Less than 1/8".
Man, I'd shove you off the stage for pointing that out.
:l2:
-
Guys there was some recent discussion about hole misalignment so I though my criticism was appropriate. Not trying to be nasty.
-
I'm laughing at what PRR said. :laugh:
-
> it's so obvious
Apple has trouble with the concept.
https://www.theregister.com/2021/06/15/m1_imac_crooked_display_issues/
-
Any suggestions or comments on the proposed layout of the Choke and the OT as shown in the attached photo?
Also, as to the Cap can, I've never installed one before and had a question(s). Can I just use the metal and phenolic oval-like "washers" for mounting, and bend the ground tab(s) to hold in place, or do I also need to use the ring mount. I have a ring mount but its too small to mount around the base of this can. If I can just use the washers, does the metal one go on the outside of the chassis and the phenolic one on the inside, if not, how are they mounted.
Thanks as always.
-
Also, as to the Cap can, … Can I just use the metal and phenolic oval-like "washers" for mounting, and bend the ground tab(s) to hold in place, or do I also need to use the ring mount.(?)
Use a proper mount.
-
Also, as to the Cap can, … Can I just use the metal and phenolic oval-like "washers" for mounting, and bend the ground tab(s) to hold in place, or do I also need to use the ring mount.(?)
Use a proper mount.
Yes.
Highest dcv in the amp connects to that can. :w2:
-
The proper way to mount that can is to bolt the metal mounting plate on the inside of the chassis. (You will not use the phenolic mounting plate). Now position the can so the twist lock tabs will go through the slots in the mounting plate. Twist all 4 tabs 90° to secure the can to the mounting plate.
Now, solder the PT HT center tap to one of the twist lock tabs. Also solder a short ground strap wire to the same tab. Put a ring lug on the short wire and bolt it under one of the mounting screws for the mounting plate. This is your power ground point.
Look at this pic to get the idea...
-
Oh yeah, here's a view of that same can from the top. Nice and neat. Those cans were very popular in the '60s.
-
Sluckey, so the clamp mount isn’t really necessary?
Thoughts on the orientation and layout of the Choke and OT?
-
Sluckey, so the clamp mount isn’t really necessary?
Thoughts on the orientation and layout of the Choke and OT?
Twist lock style cans never had a clamp. I showed you the proper way to mount that style cap. Heck, twist lock cans were so popular in the '60s that some manufacturers didn't even bother with the mounting plate. They just punched the chassis for the twist tabs to fit. Now that's really clean looking. Look at this...
http://sluckeyamps.com/hammond_2/big_guts.jpg
Choke and OT look fine.
-
Thanks. It certainly looks cleaner. I wonder why they started using the clamp?
-
Clamp style cans have been around a long time too. They are much easier to change. More secure too. The twist-lock cans can be a real PITA to remove, especially if all twist tabs are soldered! I never saw a twist-lock can in any military equipment.
-
I wonder why they started using the clamp?
those came along in the 70's when all the field engineers bitched about having to replace the dog ear'd caps :icon_biggrin:
-
Since the PT (Hammond 290BX) that I'm using doesn't have a CT for the 6.3V filament winding (see spec sheet attached), am I correct that I'll need to add a pair of 100 ohm resistors at the Indicator Lamp to create a virtual CT for this amp?
-
I ask because the PT in the Schematic/Layout that Sluckey did for me appears to be the same PT as the one I'm using and it also doesn't show a CT for the filament section of the Secondary.
-
I ask because the PT in the Schematic/Layout that Sluckey did for me appears to be the same PT as the one I'm using and it also doesn't show a CT for the filament section of the Secondary. Is that because the 100 ohm resistors are built onto the turret board? I assume so, but just want to be sure.
-
The answer you seek is on the layout. Trust your eyes.
-
Sluckey, in looking at your Layout, what type of Ground connection is used for the 1 ohm Bias Check resistors? You have a ground wire running off that connection point and going to the Power Ground Point on the chassis. Is it an insulated connection point that keeps it isolated from the chassis?
-
The black thing that the two 1Ω resistors connect to is called a tip jack. The two red things are also tip jacks. They are all insulated from chassis. That's why there is a wire connecting the black (common ground) jack to a power ground lug. They are used to conveniently monitor bias current. Just plug your meter in.
They are not needed. You can connect the 1Ω resistors directly to the power ground if you prefer. Here's a pic of the tip jacks Hoffman sells...
(https://hoffmanamps.com/MyStore/catalog/images/IMG_2293.jpg)
-
One other grounding clarification, in your Layout where you show the Preamp Ground and the Power Ground connections, there's shown a Chassis Gnd coming off each of the connecting points. Is that just to indicate that the Ground Ring, or whatever type of connector is used, is grounding to the chassis directly at that point with the screw holding the Ground Ring down? I"m sure this is obvious but just want to be sure its not intended to connect somewhere else. I think your layout shows only 2 wired ground points in the circuit (one for Power and the other for Preamp so probably no ground loop issues), other than the Power supply ground at the receptacle. Thanks.
-
yes
-
Steve, you have such patience. People here have a real fountain of information.
-
I know they do. I've gotten a lot of help on both of my projects from a lot of folks out there. Good to know where to go for the answers when you're new at this. I wish I knew more (maybe "something" is a better word) about the theory behind the design, etc., like how to know what size cap or resistor to put in a circuit and why. Thanks.
-
I know they do. I've gotten a lot of help on both of my projects from a lot of folks out there. Good to know where to go for the answers when you're new at this. I wish I knew more (maybe "something" is a better word) about the theory behind the design, etc., like how to know what size cap or resistor to put in a circuit and why. Thanks.
Learn about frequency and how to use a resistor/cap to make a low pass or high pass filters.
Read tube charts and if you do not understand what it all means, you are not alone!
Nice contributions from Sluckey, but should have the More Tremolo and more cowbell. Half of a tube not used is the sin of the Bassman, but makes a pretty nice gain stage.
I had fun, I hope y'all did too!
-
Sluckey-in the schematic/layout for the Line Out, they show a 1 uf 63 V Poly cap. I have a 1 uf 650V Poly cap, but its pretty big. I also have a 1uf 50V Electrolytic cap. Can either of these be used?
-
DO NOT USE ELECTROLYTIC! Of course you can use that huge 650v cap but that seems a bit silly. Wouldn't you rather have a proper $0.25 cap that's the size of the fingernail on your pinky? I know I would.
http://smallbear-electronics.mybigcommerce.com/topmay-tmc05-15-f-1-f/
Might even be able to find one at RadioShack.
-
Thanks for the source. I've had a hard time finding them elsewhere. I was hoping to get it finished this weekend but I also don't have the .01uf 2KV cap for the standby switch. So it looks like I'll have to wait until I can place an order for this stuff.
-
You can find all these caps at Mouser. May take some extra time to figure out how to apply filters to their cap listings. That STBY switch will work just fine without that .01 cap. Do you really even need a STBY switch with a 6V6 amp? I suspect the line out will work without that 1µF too.
-
Thanks. As to the Standby, I wouldn't have put it in except it was part of the schematic. So the hole is drilled, and I may as well use it. It seems like 2kv is pretty big. I can't imagine that much voltage going through this system. I might have a .01uf film cap that's rated at 630V just to deal with putting in something there, or might skip it. They don't have it at Small Bear. Options?
-
....Might even be able to find one at RadioShack. ...
I dunno your town; here the Shack closed a couple years ago.
https://el34world.com/Forum/index.php?action=dlattach;topic=27260.0;attach=90518
The 1uFd is not -too- essential? Just prevents ice-pick?
Anyway 470/47 is real generous. 4k7/470 would be fine even sending signal across the stadium. Then the cap could be 0.1uFd, which is a common value in tube amps. Or 10k/1k and 0.05uFd.
-
If I put a .01uf 630VDC film cap on the Standby Switch, just to make me feel better until I get a proper size, will it get blown the first time I turn on the amp? Should I just leave it out for the time being?
-
The cap across the STBY switch is there to deal with arcing/sparking of the switch contacts. An arc can contain voltages much higher than the voltages you can measure with your DMM. Those caps were never used in the old '50s/'60s vintage amps we love. As I said, you don't need it. You could just put a pin in it , build the amp, and order the cap whenever it's more convenient. Or never. :icon_biggrin:
You're building a clone of a modern recreation of an old amp. They are using modern components. Gonna have to turn to modern sources for those items. Hoffman ain't gonna have them.
But an old radar tech just might have them. :icon_biggrin: Want these? It'll cost you a beer. Nickle not included. Send me a pm with your mailing address if interested in this new old stock.
-
If I put a .01uf 630VDC film cap on the Standby Switch, just to make me feel better until I get a proper size, will it get blown the first time I turn on the amp? Should I just leave it out for the time being?
Maybe not the first time, or even the second, but some day... A 630V cap ain't gonna make you feel better. I know you. It's gonna worry you to death! :l2:
-
You’ve got me figured out alright!
-
While I'm waiting for the cap, I guess I could avoid the arcing by leaving the Standby turned OFF, ie. in the Play position.
-
Not interested in my offer? OK, back in the parts bins.
-
Sorry Sluckey, I thought you were just kidding. Sure, I'd appreciate them if the offer is still outstanding. You were up mighty late last night. I"ll pm you my address and let me know what I owe you. Thanks.
-
Sluckey, I finished the wiring today--WHEW--, and started going over it with my DMM. I have a question about the readings I'm getting on the grounding on the Footswitch, Int. Speaker and Ext. Speaker jacks. When I connect the meter's black wire to the chassis, and probe the jacks with the Red wire, I get a continuity reading when its touching the GND pole on the jack as well as when I touch the Tip pole of the jack, for both the internal and external jacks. Is this because the OT Secondary Black wire is wired to the GND pole of the Internal Speaker Jack and so it shows up as being in continuity on both of the GND poles and the Tip poles. I am using a Switched Jack as you show in your Layout drawing, and nothing is connected to any of the Switch Poles of the jacks. OR, have I wired something incorrectly? Thanks.
-
Hmm, I wonder what it could be? :think1: I bet all your other amps read just like this one. What could cause that? :icon_biggrin:
-
Well, I first thought it was because it’s a switched type jack. So I put a plug in the jack to release the tip from the ground switch, but it still read as grounded. Does it relate to the NFB looping back in the circuit to ground?
-
I get a continuity reading
can your meter read milli-ohms?
think about what's "in-circuit" that produces "continuity"
-
Come on buddy. You can do this.
-
Line up a whole bunch of output transformers. Measure the secondary winding DC resistances.
Or if you haven't got a bunch of output transformers to measure, try googling: 'DC resistance across output transformer secondary winding' or something like that
Or better still, try this: https://letmegooglethat.com/?q=DC+resistance+across+output+transformer+secondary+winding&l=1
-
So what I’m actually seeing when I have the meter in continuity beeping mode, is not resistance, but impedance?
-
So what I’m actually seeing when I have the meter in continuity beeping mode, is not resistance, but impedance?
Not impedance. Impedance is ‘AC resistance’ and you need to measure the pr:sec VAC ratio to calculate impedance.
When you meter a winding for continuity, it’s using the closed circuit resistance between one end of the winding and the other, i.e. DC resistance.
-
...I have the meter in continuity beeping mode...
Continuity testing is rarely useful when working with any electronics equipment. There is a resistance threshold that triggers the beep. In some cheap meters, that threshold may even be as high as 1KΩ! This means that everything above 1K is considered "no continuity" and anything below 1K is considered "continuity". Better meters have a lower threshold but it will still be some none-zero number. Some meter manuals will state what the threshold resistance is. You can discover what your threshold is by connecting progressively larger resistors to your meter. As long as you are below the threshold the meter will beep, but as soon as you connect a resistor that's greater than the threshold, the meter will not beep.
I'm sot saying the beep is useless. It can be very useful, just not inside an amp chassis. Knowing actual resistance is much more valuable inside an amp.
-
I guess I was in the neighborhood in my Reply #168 above when I posed whether the OT was the culprit. So it is, but not completely for the reason I was suggesting. Thanks as always for the learning experience. As you have observed, I'm not satisfied with a "just do it" approach, but rather "why am I doing it". Thanks. I realized that the reason I hadn't seen this in my last build of the Mag 10A-Revibe Mod of Sluckey is because it doesn't have an OT! It just goes to the output jack which connects to the input of my Gretsch or Champ power amp that I have. I'll be going over the circuit a couple times today to make sure everything is wired up correctly before plugging it in and starting it up. I may way to do that tomorrow.
Any suggestions on startup testing, etc?
-
Continuity simply means you have a closed circuit, therefore if you have no continuity you have no circuit.
MY METER READS CONTINUITY FANTASTIC, SLuckey. Invest in a Fluke why don't ya. Doesn't tell you much tho, but it does have a piezo speaker and it is annoying. Use you resistance settings and if it changes in ANY way when you probe, you have continuity.
That capacitor is way too important to the tone of the amp. Go and get one rolled on the inner thigh of a polynesian 18 year old chic.
Yes, Start it with no tubes on a Light Bulb Limiter if you do not have a variac (Autoformer) and if you do not have one, consider finding one used. Take voltage readings and they will be high, no load from the tubes. It will still let you know if it is wired correctly. Post your voltages. Let's see them.
Then maybe we willl give you a break to watch a movie!
-
In addition to Ed's advice, while connected to a light bulb limiter and with no tubes plugged in, adjust the bias pot for maximum negative voltage on pin 5 of both 6V6 sockets. Then connect an appropriate speaker and put the tubes in. If the light bulb is not glowing very bright, then unplug the amp and put the light bulb limiter under the bench (because you no longer need it). Now plug the amp directly into the wall and turn it on. Be prepared for a loud squeal. If you get the squeal unplug the amp from the wall and swap the OT plate leads. If no squeal measure the voltages on every filter cap. Then play some guitar.
-
Thanks everyone. I've got a Variac and I also built a Light Bulb limiter. So good to go there. Also, I found I did have the .01 uf 3KV cap that I purchased for this build. It wasn't labeled, but Sluckey's photo told me what I should be looking for and I checked out my invoices and found it. So that's now installed on the Standby Switch. Sluckey should I have a dummy load plugged into the Speaker Jack when I'm setting the initial bias you describe?
-
Sluckey should I have a dummy load plugged into the Speaker Jack when I'm setting the initial bias you describe?
Well, if you follow my instructions you will not have any tubes plugged in. So what do you think? :icon_biggrin:
When you finally plug in the tubes you want a real speaker connected so you can listen for odd sounds like a squeal.
-
DUhhhhhhh-Ok, that was a brain fart!
-
I like to use my meter to measure Resistance of the power supply WITH NO POWER applied, NO tubes reqired
Ground the Neg lead, put the lead on The 1st Tap, usually A and watch, the resistance will start going up real fast and hopefully settle somewhere in the > 100k, howfully Meg range
NOTE: you will put a charge on the caps doing this, but not enough to bite...hard :icon_biggrin:
This essentially the same as the limiter without volts, anything < 100k might indicate a problem
-
Shooter what are saying here: “ howfully Meg range” ?
-
Shooter what are saying here: “ howfully Meg range” ?
That's weed speak for "hopefully Megohm range". :laugh:
-
:laugh:
that happens at 6:20pm local time, I'm old :icon_biggrin:
-
Shooter, where are you? In the UK?
-
:laugh:
I'm a long-haired country boy in farm country MI, passed 30yrs, plus 0-18. Indiana, Virginia, and a year north of the bay bridge in San Francisco
-
Shooter it peaked at about 40 Meg Ohms.
-
Sluckey, the only amp I've had to deal with the bias was the Tweed Champ which is Cathode fixed bias as you know. So no pot involved. Could you walk me through how to bias this one? I have access to a Eurotube single socket bias probe that measures DC Milliamps if that's helpful.
-
All you need is a meter and calculator. No fancy bias probes needed. I already told you how to initially set the bias pot. Now read this. Maybe even save it in your favorites...
http://www.duncanamps.com/technical/lvbias.html
-
Will do. Thanks.
-
Power problem. I’ve got 120 VAC to the on/off switch showing on my DMM at the 2 tabs of the SPST switch in the OFF position, and when I turn the switch to ON, the Power Lite comes on but there’s NO voltage at the switch tabs. If I disconnect both wires, and put the Ohm meter on the tabs, it shows OL when off, and 0.000 when On. What have I messed up? All the wiring looks correct.
-
Sorry, ignore that post. I am getting AC power out of the switch and int the PT. I don’t have any tubes installed. I’m not getting any DCV readings at any of the nodes on the cap can regardless of whether the Standby is on or off.
-
I am getting AC power out of the switch and int the PT. I don’t have any tubes installed. I’m not getting any DCV readings at any of the nodes on the cap can regardless of whether the Standby is on or off.
Hmm, I wonder why. What would it take to rectify that problem?
More important... set the bias pot for max negative voltage at pin 5 of the output tubes. Both of them.
-
I don’t know what happened to an update I sent about 45 minutes ago, but I’ll recreate it.
I checked voltages at the rectifier socket, and I’m getting voltage at all pins except for the yellow wire at pin 8. The other yellow wire at pin 2 is showing 5 VAC, and each of the red wires show 350 VAC. So no power is getting to the OT node on the cap can. Is it a bad PT winding?
-
I checked voltages at the rectifier socket,
with OR without a tube?
-
I am getting AC power out of the switch and int the PT. I don’t have any tubes installed. I’m not getting any DCV readings at any of the nodes on the cap can regardless of whether the Standby is on or off.
Hmm, I wonder why. What would it take to rectify that problem?
More important... set the bias pot for max negative voltage at pin 5 of the output tubes. Both of them.
Haha this is just the best thing I've ever read pertaining to tube amp troubleshooting
Gotdang, gotta pop open a cold one after that
-
Shooter—No tube—
As to Sluckey”s “rectify” hint, I actually went there before seeing his hint-for what that’s worth! Still, my question is whether there’s a problem with the PT since I’m getting voltage on one yellow wire going from the PT to the rectifier socket, but not the other yellow wire. And it’s the “other” wire that’s feeding the OT at the cap can.
-
Show us a picture of your rectifier socket.
-
I checked voltages at the rectifier socket, and I’m getting voltage at all pins except for the yellow wire at pin 8. The other yellow wire at pin 2 is showing 5 VAC, and each of the red wires show 350 VAC. So no power is getting to the OT node on the cap can. Is it a bad PT winding?
You cannot measure the 5VAC at pin 2 or 8 if you have one meter probe on chassis. The only way to accurately read the 5VAC is to put one probe on pin 2 and the other probe on pin 8.
-
Here’s a photo of the rectifier socket.
-
Your wiring is fine. Your method of checking the 5VAC is suspect.
-
Ok, so that I’ll have something to dream about tonight, since I was told to check and write down the voltages before I install the tubes and let the group know the readings, shouldn’t I be seeing voltages other than just at the rectifier socket? If so, what would you expect me to see? Thanks.
-
I've asked several times about the bias voltage on pin 5 of the output tubes. I ain't answering any more questions until you answer my question.
-
That’s fair. I’m burned out for tonight, but will get that posted in the morning. Thanks.
-
Ok, so pins 2 & 8 read 5.41 VAC
When the bias pot is adjust to the maximum negative VDC:
V-4 reads -0.901VDC—90mv
V-5 reads -0.902VDC
-
When the bias pot is adjust to the maximum negative VDC:
So what is that voltage?
-
- 90.1 mv
-
No, that should be -901 mv! That doesn’t make sense. I’ll check the auto-scale on my meter.
-
Where is your meter connected when you measure that 901mV?
-
The red probe is on Pin 5 where the 1.5K resistor attaches, and the black is to the chassis.
I checked it with another meter and read the same thing.
Hopefully you are going to tell me that I've got the black probe incorrectly connected to chassis.
-
Nope. I'm gonna tell you that's a problem and you will have to troubleshoot to find what's wrong. Plug the amp straight into the wall and recheck pin 5 voltage. If it's not something like -30v to -50v, then the troubleshooting starts. Check for -30v to -50v (or more) at the junction of the diode and the first bias cap. If no good, set your meter to read AC volts and check for approx 50VAC on the other side of the diode. If not good, check for approx 50VAC at the junction of the 470Ω resistor and the wire from the PT. Report findings. Somewhere along this troubleshooting path you should have found the problem. Could be wiring error or bad/incorrect component.
A hi-rez pic showing the bias pot end of the board would be a big help right now.
-
Here’s the photo.
-
Ok, here are the results.
1. With amp plugged directly into wall receptacle, and black probe on chassis, and red probe on Pin 5 the reading is -0.936 VDC.
Black probe connected to chassis and Standby ON, ie. not in Play position, for the following measurements:
2. At the junction of the diode and the first bias cap (470 ohm 3 W) it reads +22.83 VDC.
3. It reads +0.001 VAC on the other side of diode.
4. At the junction of the 470 ohm R, and the Blue 50V PT wire, it reads 0.198 VAC.
-
Sorry, #4 above does read 54.7 VAC not 0.198.
-
Replace that 470K 3W resistor with a 470Ω 3W resistor!
-
or just paint the 2 yellow stipe brown n call it good :icon_biggrin:
-
Replace that 470K 3W resistor with a 470Ω 3W resistor!
While you're at it, do the same for the two 470K 3W resistors on the tube sockets.
-
Wow-I must be going blind. But I did have 470K 3W resistors for some reason. Why I don't know because there are none in this layout. I must have ordered them wrong in the first place. So all I have on hand is 470 ohm 2W resistors. Should I use those or wait for a new order arrives? Thanks as always.
-
So all I have on hand is 470 ohm 2W resistors.
Those will be fine in all three places.
-
Wow-I must be going blind.
fwiw;
I always ohm every resistor before I install it.
The CCP is real bad at brown, red, and orange
-
I think things are going right.
Pin 5 is showing-59.3/-59.2 VDC max. Neg bias.
51.0 VAC at the diode/first bias cap.
-61.2 VDC or 0.00 VAC on other side of diode at the 47uf cap. IS THIS CORRECT?
52.6 VAC at junction of 470 ohm resistor and blue PT wire.
Issues or good to go with next phase of startup?
-
:thumbsup: Much, much better. But before we go on, let's fine tune the negative voltage on pin 5. Still no tubes plugged in. We really want the bias pot to be able to swing from approx. -25v to approx -45V for 6V6 tubes. So, we need to experiment with that 470Ω resistor on the board. That resistor is commonly referred to as the bias range resistor and right now it's too little so pull it out and set aside. Now temporarily connect a 1K resistor in that spot. Gator clip leads make this easy. Check the bias voltage on pin 5 and adjust the bias pot from one end to the other and note the max and min negative voltage values. If the range is now close to -25 to -45, you're done. Permanently install the resistor and set the bias for max negative voltage on pin 5.
If you still don't have a proper range, grab another resistor and repeat. A larger resistor will reduce the voltage and a smaller resistor will increase the voltage. You can use 1/2W or 1W for testing but once you have nailed down the value you need, get a 2W or 3W for the permanent resistor.
Your 6V6s will probably want about -35 volts for proper operation. I chose the -25 to -45 range so the pot will sit at about halfway for -35v. This way you can adjust hotter or cooler as you wish. When you get this nailed down report the resistor value and the range of voltage on pin 5. Leave the bias pot set for max negative.
Don't proceed any farther until this step passes QC. I'll be close to the forum for the rest of the evening.
-
Thanks. I’ll work on this in the morning. Thanks again.
-
Sluckey I’ll check the range again tomorrow before I do anything, but I think the low reading was about -25 VDC.
-
Sluckey I’ll check the range again tomorrow before I do anything, but I think the low reading was about -25 VDC.
The max negative voltage is applied to a voltage divider consisting of a 10K pot and a 10K resistor. This means the minimum voltage will be half the maximum voltage. IE, if the max is -60v, the min will be -30v. But if we can drop the max down to -48, the min will be 24, and this is a much better range for 6V6s. The bias voltage for 6V6s will likely be about-35v when tubes are biased to about 70%. This puts the bias pot at about the middle of rotation, giving you plenty of adjustment to bias hotter or cooler if you like.
-
What Luck! I have a 1.5K @ 2W that brought it to -25.98 to -48.6 VDC! So I’ve installed that one.
-
What Luck! I have a 1.5K @ 2W that brought it to -25.98 to -48.6 VDC! So I’ve installed that one.
:thumbsup: Set the bias pot to -36V at pins 5 of both 6V6s.
Next step... With amp plugged straight into the wall and amp in ***STANDBY*** mode, plug in all tubes, turn the amp on, and insure you can see ***ALL*** filaments glowing. 12AX7s can be hard to see so dim the room lights if necessary. Now measure the filament voltage at V1 with meter connected ***ACROSS*** pins 4,5 and 9. Should be 6.3VAC.
Next step... Unplug the amp from the wall and plug it into the light bulb limiter. Connect a proper speaker. Turn the amp on and put the standby switch in ***OPERATE*** mode. If no bright light and no smoke, turn the amp off, unplug from light bulb limiter, and plug directly into the wall. Turn the amp on. If no loud squeal, smoke, or red plates, measure the dc voltage at all four B+ nodes. Play a little guitar. Report the voltage readings and any unusual sounds, and tell us how the guitar sounds.
-
How can I "Set the bias pot to -36V at pins 5 of both 6V6s"--the word "both" confuses me since I only have 1 pot. I can read the bias voltage on each of them, but I can't separately adjust them--can I?
-
You're that guy that all instructors just love. :l2:
OK, let's make this easy. Set the bias pot to -36V at pin 5 of any 6V6. You get to chose which one. Now check the voltage at pin 5 of the other 6V6. I'm betting it will be -36v too.
-
How can I "Set the bias pot to -36V at pins 5 of both 6V6s"--the word "both" confuses me since I only have 1 pot. I can read the bias voltage on each of them, but I can't separately adjust them--can I?
No you can't adjust the -bias separately, but, when you adjust the -bias pot, it will adjust both power tubes -bias at pin 5 at the same time.
Because both tubes pin 5 are fed from the same source.
They will be the same -dcv or very close.
-
Thanks. Yes I know! I guess that’s the retired lawyer still hanging around in me. Every word has a meaning and sometimes it’s important.
-
I should have guessed. You guys speak and write in such a cryptic manner that the only people that understand is another lawyer. I'm gonna see if jjasilli will rewrite my responses so you can understand them. :l2:
-
I see the filaments glowing on the 3 12ax7’s and the GZ34 but NOT on either of the 6V6’s. I’m reading 6.43 VAC on the 12ax7 filaments.
-
I know why. I didn’t wire pins 2 & 7 on them to the 6.3 V filament wire. You don’t need to say anything.
-
I see the filaments glowing on the 3 12ax7’s and the GZ34 but NOT on either of the 6V6’s. I’m reading 6.43 VAC on the 12ax7 filaments.
Too late. jjasilla, you want to take this one?
-
Ok, everything lit up, no smoking. Then I went your next step and I got the high pitch squeal. I assume from what you said earlier in this process that I need to reverse the OT Secondary wires at the Internal Speaker Jack. Correct?
Can you achieve the same temporary fix by reversing the wires on the speaker?
-
And I did get a low warm glow on the light bulb limiter.
-
Sorry, I meant switch the OT leads at Pin 3 on each of the power tubes.
-
Sorry, I meant switch the OT leads at Pin 3 on each of the power tubes.
Yes, you can switch either the OT primary leads (pin 5) or the power tube control grid (grid 1) (pin 3) leads.
Edit; Pin 3 = power tube plate, pin 5 = grid 1. If you put the grid stopper across pins 1 and 5 then swap the wires at pins 1.
-
If you want to test it 1st, just disconnect the -feed back wire. If that stops the squeal, then swap leads at the power tube.
-
Sorry, I meant switch the OT leads at Pin 3 on each of the power tubes.
That's correct. Looking at your pic I'd say you're gonna have to stretch that brown OT wire a bit. Another lesson to learn... Don't cut the OT primary plate leads until you are sure the NFB phase is correct.
There is another easy way to accomplish the same thing and it will be very easy to do on your amp. Locate the two green wires that go from the board to pin 1 of each 6V6. Swap those two green wires on the board. Easy, peasy.
-
I'm gonna see if jjasilli will rewrite my responses so you can understand them.
.........as wsscot is learn'n to translate the Tech language to Lawyer speak :icon_biggrin:
-
I don’t know which language is worse to learn. I guess once you learn it though it’s no big deal. If I can learn these 2 languages, then maybe there’s still hope for me learning Italian!
-
Any joy with the amp?
-
He must be jammin. :icon_biggrin:
-
more like :sleepy2:
-
Sorry guys, but we had some thunderstorms and lost power for awhile, so will be getting to it this morning. Will keep you posted. Fingers crossed.🤞
-
What part of the country are you in?
-
Charlotteville VA
-
Great news!! Everything checked out OK. No squeals, pops, smoke, etc. Here are the VDC readings on the Cap Nodes:
A: 437
B: 383.1
C: 359.9
D: 431
OT: 441 (PLAY)/ 499 (STANDBY)
Looks like B & C are a little higher than on the Mag Twilighter Schematic, which I understand would be different than mine:
A: 422
B: 297
C: 269
D: 424
I'm taking a lunch break and then see what the guitar sounds like and will report back.
Any comments?
-
What is the voltage on pin 8 of each 6V6? Looking for two numbers.
-
.... OT: 441 (PLAY)/ 499 (STANDBY)...
Is that first cap good for 500V??
In fact: in "standby" there should not be ANY voltage at the OT? Or have we moved on to another plan?
-
He's talking about how I have the labeling on the cap can. My layout has the first node cap labeled as "OT". My original layout did not have a STBY switch so I connected the OT ( and choke, and the rectifier) directly to the first node cap. Later when he decided he needed a STBY switch, I installed it in the usual manner, but I did not change the "OT" label on the cap can. It's all my fault! The actual OT lead is no longer connected to the cap labeled "OT".
-
Sluckey I didn’t have any 1 ohm resistors for the bias test points, so I wired pin 8 on each 6v6 to a insulated post that I can use when I get the resistors, and then ran that connection to Power Ground point. So in reading Pins 8, with Standby Off, black probe on chassis, and red probe on Pin 8, I read 0.4 MV DC on each one.
As to the cap can, it’s rated for 525 V.
-
I’m also using right now a dummy load resistor instead of the speaker.
-
You need a speaker to tell if it's squealing. What does the amp sound like with guitar?
-
Ok. Guitar sounds great. No hum, hiss, noise or anything. Quiet as a church mouse. Mid seems to have greater effect than Bass. Only obvious issue is NO TREMOLO.
-
Sounds like progress. This is just to let you know you are not the only one who took Ohms for kOhms. Keep on keeping on.
-
using a foot switch?
-
Yes using a foot switch. It came with a RCA jack and I am using an adapter to 1/4”.
-
Don't need a footswitch. Tremolo is enabled by default.
-
The Intensity seems to slightly increase the sound, so it’s probably working, but the SPEED doesn’t. I also tried a different foot switch but no different.
-
The Layout shows no wire connection on the Far Right tab on the Speed pot-looking from behind. Is that correct?
-
The Layout shows no wire connection on the Far Right tab on the Speed pot-looking from behind. Is that correct?
yes
Measure voltage on V3, pins 1, 2, and 3. When measuring pin 1 watch it for several seconds. If the tube is working, pin 1 voltage will fluctuate at the speed of the tremolo. This fluctuation will just appear as an erratic reading on a digital meter.
Post a hi-rez pic showing the area of the board that has the tremolo components and the tube socket and the INT and SPEED pots.
-
Pins 1, 2 & 3 respectively. No fluctuating on any of them. Pictures attached. May have to send separately.
202.3 VDC
1.3 mv DC
1.83 VDC
-
Another photo.
-
I switched tubes, but no change.
-
And you have 2 RCA jacks by the trem tube that are not wired up.
-
See that 47K that's parallel to the yellow .01 cap? Remove that resistor. Got trem now?
-
Yep! Yea!
-
Sorry, so is it too big, or too small or not needed at all?
As to the unwired RCA jacks, that’s for future expansion for a reverb if I decide to add one.
-
Sorry, so is it too big, or too small or not needed at all?
Not needed at all. I've seen a lot of tremolo circuits, including lots of Maggies, and not one ever had a resistor across that cap. I wonder if that resistor is really in the original?
LMK if you find you need a stronger tremolo. I have a simple mod to make it so.
-
Not needed at all. I've seen a lot of tremolo circuits, including lots of Maggies, and not one ever had a resistor across that cap. I wonder if that resistor is really in the original?
I was wondering why that R was there too. I was going to ask you about.
-
Sluckey thanks for all your help. I’ll have to get used to the sound of the Trem as it currently exists, and may want to change it. I definitely like the Mag Vibrato better. The speed dial goes from Fast to Slow Trem on this amp. Can I just leave the center wire on the Speed pot where it is, and reverse the position of the other wire in order to have it go from Slow to Fast? It just seems more intuitive to me.
Also, how did my voltages look on the Nodes? Any issues on B and C that were high vs. Mag. schematic?
-
This has to be the greatest post in a long, long time, l still can’t stop laughing. Fantastic, post. :l2:
-
The speed dial goes from Fast to Slow Trem on this amp. Can I just leave the center wire on the Speed pot where it is, and reverse the position of the other wire in order to have it go from Slow to Fast?
Yes, except the other wire is a resistor. Just move the resistor from the left side of the pot to the right side of the pot.
I'm happy with the voltages.
-
Willabe attached is a copy of the schematic that Magnatone gave me. The 47k is definitely in the circuit, and the wiring looks a little different than Sluckey’s-but what do I know as you all can tell by my posts. It looks like the grounding may be a little different. I’m sure someone here can appreciate the subtleties.
-
Sluckey, thanks again for your help and time. Much appreciated. Best.
-
when you swap the resistor, WRAP it or at least bend it 180. a good solder joint relies on a good physical bond.
-
The 47k is definitely in the circuit
Of course it's on the ***SCHEMATIC***. That's why it was included in my drawings. My question is the resistor actually inside a real amp.
-
I notice several other, more essential, parts have the "**" notation for "do not insert".
I think this part may have been drawn after happy hour.
-
parts have the "**" notation for "do not insert"
I never understood the significance of that notation. Didn't think too hard on it.
-
I'd like to see a pic showing the entire top side of the chassis and another pic showing the entire inside of the chassis.
-
Sluckey, I was looking at Rob Robinette’s bias calculator for 6V6’s and it shows a bias setting of around 25 mA. I used a friend’s bias probe on 1 of the 6V6’s this morning just to see how it works, and it reads a bias of 37.61 mA for that tube. Is my bias set too high for the 6V6’s I’m using?
-
Looks like it's too hot. Adjust the bias pot for 25mA using the bias probe. Then switch to the other tube and see what the current is. If it's higher than 25mA readjust the bias pot for 25mA on this tube.
This is only half of the process. Now you have to measure the voltage on pin 3 and multiply times the .025 current reading to get the static dissipation in watts. This is a good time to read that bias link you bookmarked.
-
I bought a set of matched tubes, and used the bias probe on them, and got them arranged so that they were reading 25.14/25.6 mV! So I used Robinette’s calculator, and based on the plate to cathode voltage it has of 447 VDC, I could increase the bias. So settled on 29.65 mA. The pin 3 plate voltage read 445, so I get a little over 13 watts. So seems good to me.
-
I bought a set of matched tubes, and used the bias probe on them, and got them arranged so that they were reading 25.14/25.6 mV! So I used Robinette’s calculator, and based on the plate to cathode voltage it has of 447 VDC, I could increase the bias. So settled on 29.65 mA. The pin 3 plate voltage read 445, so I get a little over 13 watts. So seems good to me.
What is the voltage on pin 5 of the 6V6s with this setup?
-
V-4 is -39.79
V-5 is -39.7
-
I've been enjoying playing the new amp! Has a really clear tone to it. Here are some first impressions after a couple days of playing it:
1. The Treble tone control has great responsiveness as does the Mid tone control pot.
2. It seems like the Bass tone control doesn't have much affect on the sound. You have to turn it up really high to get any noticeable change in tone, and then there's very little range or bass added to the sound. Sluckey added the Mid Pot for me, and in his schematic he shows a 10K Pot for the Mid, but makes a notation "Use 100K-A for Mid-Raw". I don't know what this distinction means. His layout shows a 100K-A pot, so that's what I installed. What's the tonal difference in using the 10K vs. the 100K? Would that be the reason for the limited response on the Bass tone control?
3. The Bright switch, which I added in the form of a toggle switch, really takes the upper frequencies to another level. Sometimes more than I need, but its a good option to have.
4. After using the Tremolo for a couple of days, I've gotten to really like it. It has a sound different than any other tremolo I've ever heard. I some ways its similar to the Mag Vibrato. A friend has an Original 1965 Deluxe Reverb and a 1978 Princeton Reverb with no pull pot on the Volume knob, and he's into recording in his studio, and he says this Tremolo is nothing like any Fender he's ever had or heard and really has a neat distinctive sound. Yes its definitely a keeper and is great to have as an option to my Vibrato.
5. The amp definitely has plenty of Volume, so no worries there. I can keep Volume pot at around 8 o'clock and have plenty of sound at home.
6. The 2 input jacks have a noticeable difference in hi frequencies. So its another great option that is meaningful and may be useful to a player.
7. I haven't been able to test out the Line Out yet since I need to order a cap and a resistor that I didn't have. I also will install the Bias test point 1 ohm resistors after I order those parts.
8. I guess a final comment is how quiet and noise free the amp is. No hum, buzz, static, etc. I think maybe that has to do with the grounding scheme Sluckey shows on his Layout. There are really only 2 ground points, other than the power source line input to chassis. He has a Power Ground and a Preamp Ground. So seems to me that's going to keep it quiet.
I had a lot of stops and starts in doing this build. I had to resolve a chassis build design that took time and created a delay. Then because of the way I wanted the front of the chassis to look in the cabinet, I had a long delay in working out the specs on the cabinet dimensions and then the build. I couldn't have the cabinet built until I got the chassis so I could send it to the cabinet guy. Building the turret board was pretty much a breeze. Sluckey included a turret layout which I modified slightly, in part to include empty space on the turret board to add future turrets for a Reverb addition. Hoffman turned that around in a day, and I loaded that up in a couple days, but then it sat waiting on everything else. I also had ankle surgery that left me out of commission for a couple weeks. So with all the starts and stops, I felt a bit disorganized in comparison to the Mag10 Mod-Revibe build that I did last year, and I'd like to think that's part of the reason for those stupid mistakes I made with some of the wiring. But I'm very pleased with the final results. Maybe a few tweeks left to try, with the assistance of you guys.
Thanks again for all your input. Best.
-
Good review. Glad you're enjoying the amp.
The mid/raw pot works like this... The first half of rotation (0-5) behaves exactly like a standard Fender 10K mid pot. The second half of rotation (5-10) actually begins to "lift" the tone stack out of the circuit. Not entirely like a switch, but the effect is very noticeable. You should hear the amp getting louder (because the tone stack is not attenuating the signal as much) and the sound should take on a raw sound (not as refined as the standard Fender tone stack). Useful if you like a little hairy growl to your sound.
To properly evaluate the operation of the Bass and Treble controls, you should keep the Mid control under half rotation (out of the raw control area). The Bass may have more effect with the mid set to about 2 or 3.
If you find you don't care for the raw sound, just replace the pot with a 10K-A pot. This will spread the mid control over the entire sweep of the pot, just like a Fender TR.
-
Are you gonna show us some pics?
-
Sure-I've got it installed in the cabinet, so you can't see the chassis, but I"ll send a cabinet view now, and will send the chassis when I pull it to install the Line Out components. I still have to label the front panel and maybe get one made for it.
-
Still waiting for some parts to finish the Line Out and add the Bias test points. Maybe they'll come this weekend.
Sluckey, if I decide at some point to add on-board reverb to this amp, can I take the section I've outlined in yellow on the attached schematic of the Mag Twilighter that you used for this Mod, and just insert it in the circuit where it comes off the V-1 Tube? Are there any changes needed? I know you changed a cap and a couple resistors when you did the Mod, but didn't know if some more changes to the circuit would be needed to add back in the Reverb section from the Mag Twi.
And this is just a general question about Schematics and Layouts. Over the last year or so, as I've been working on these projects, and looking at schematics and layouts, it seems that the Schematics are always drawn from left to right, input to output. But when the Layout is drawn, its right to left, input to output. It seems like it would make more sense to draw the Layout in the same direction as the Schematic. I'm sure the answer is obvious to someone experienced in this, but I'm not there. Can you give me your thoughts? Thanks.
-
But when the Layout is drawn,
when the board is installed, then the chassis gets flipped upright, which way is it "drawn" :icon_biggrin:
-
if I decide at some point to add on-board reverb to this amp, can I take the section I've outlined in yellow
You also need the section outlined in red. See attached.
But when the Layout is drawn, its right to left, input to output. It seems like it would make more sense to draw the Layout in the same direction as the Schematic.
It's a matter of personal preference. I draw my layouts the same as Fender did. Hoffman draws his layouts left to right, input to output. You can rotate my layouts 180° if you prefer, but the writing will be upside down. :l2:
-
I draw my layouts the same as Fender did
I've find something interesting to be read
https://www.ampbooks.com/mobile/amp-technology/layout/ (https://www.ampbooks.com/mobile/amp-technology/layout/)
Franco
-
Sluckey when I built your Mag 10A mod last year for my Revibe Mod, that circuit used a diode rectifier and this amp uses a tube rectifier. Why the difference?
-
Look at the original schematics and you tell me. :icon_biggrin:
-
Well I guess the difference is that the PT in the Mag 10A did not have the Yellow wiring for the Rectifier tube and so the rectification was done with the diodes, where the Twilighter PT does have that wiring available for the rectifier tube. If that's the reason, then I guess the ultimate issue is why use one type of PT vs. the other? Cost?
-
why use one type of PT vs. the other
Now you're getting into the Tea leaf reading section :laugh:
The Valve Wizard (http://www.valvewizard.co.uk/bridge.html)
you can glaze over the math, (for now).
The tea leaf part;
some musicians want the "Tonal" sound you get from sagging a tube Rect. Others want Stiff, no sag so it becomes a "preference", not an electrical function. Still others want "as close as possible" to an amps original design
-
I guess the ultimate issue is why use one type of PT vs. the other?
Manufacturers choice.
As for my choices, well I just followed Magnatone's lead. Your revibe unit has no power amp. Everything operates in class A so tube rectifier sag does not exist, so SS is a better, cheaper, more reliable rectifier choice. As for your twilite with a fixed bias power amp, rectifier sag is a real factor. But my reason for using a tube rectifier is because Magnatone did. Magnatone used a tube rectifier because that's a marketing hook for their new line. And since Magnatone simply copied the Fender Deluxe Reverb which has a tube rectifier, they used one too. So, the young non-tech git players can gush and say, "My tube rectifier sounds so much better. It has real girth and length. Can't you hear it?" :l2:
-
Well at least I could ID the distinction between the 2 circuits and PT's!! HAHA. So at the end of the day, there's really no technical reason requiring one over the other, just "preferences" of some sort.
-
Well at least I could ID the distinction between the 2 circuits and PT's!!
Did you ID with your eyes or ears?
-
Eyes!
-
I finally got the last resistor I needed for the Line Out and got that installed today. Got the Bias Test Point resistors installed the other day. So as far as I know everything is now finished and operational. I'm going to leave the Mid Pot as is for the time being and see how I like it, or if I want to change it to the 10K as Sluckey said was a way to get more bass. At some point I may come back and add the Reverb circuit. I didn't do it to begin with because a lot of tube amps I've heard, or heard of, seem to have hum that's associated with the Reverb when its turned on. I don't know if that's inherent in the design, or the build, or what. But this amp is quiet as can be and I'm very pleased.
Attached are some photos of the final build. Thanks again everyone for your help. Best.
-
At some point I may come back and add the Reverb circuit.
I don't think you have room for tube reverb on/in that chassis.
I didn't do it to begin with because a lot of tube amps I've heard, or heard of, seem to have hum that's associated with the Reverb when its turned on. I don't know if that's inherent in the design, or the build, or what.
I don't think that's true. As long as they put it in right, layout, space, lead dress.
I don't think Fender BF amps with verb are any nosier than non-verb amps. Very minimal difference if any.
-
Sluckey did you ever do any Visio Layouts for any Mags with Vibrato and Reverb? If so, could you post maybe the one(s) you like best. I might want to try one of them sometime. Best.
-
My L'il Maggie M2 is the only layout I have. It's on my website.
-
I’ll check it out. Thanks
-
Just as an update, I decided to change out the 100K-A Mid Pot for the 10K-A. I like the 10K much better. The RAW sound when the 100K got past mid-way wasn't something I liked, and the 10K gives me a lot more control over the Mid and Bass frequencies--or so it seems. Anyway, I think I'll be sticking with this. Otherwise the amp is doing great. Very pleased with it and Sluckey's design mod to the current Magnatone Twilighter circuit. Thanks again to everyone.