Here is my latest build, an ODS inspired 4x6L6 100 watt amp.
The chassis was purchased from Erwin. He was out of chassis with 3 preamp holes but had some with 4. It is raw aluminum with front and rear panels rather than his normal powdercoat with stenciling. IMO a nice chassis that has worked well. The only modification was a hole I drilled for an extra switch.
My biggest issue was what to do with the 4th preamp hole? I considered several options...
Cover plate. Naaa, too easy.
A single tube reverb. Looking into this option I found others had tried this in various amps and the general consensus was that a single tube spring reverb was weak and disappointing. I don't want solid state.
A CF similar to what a SSS has. Very tempting, definitely a contender.
Built in FX Loop buffer. Back panel has holes and labels for send and receive level controls. Another contender.
After much consideration I decided on the FX Loop Buffer. With an added *(semi) true bypass relay. The relay is positioned so when in bypass there is very little disruption to the signal path. Relay is controlled via a switch added below the OD and PAB switches on the back panel. Buffer values were taken from drawing modified by Martin Manning (Thanks Martin!!). In fact most of this was cut/paste from existing drawings on TAG.
The FX relay is wired in such a way that the Loop always has an input so the FX out can be used even during bypass. This splitting of the signal is the *(semi) portion above. Loop has send/receive level controls with bright push/pull on the receive pot.
The boards provided by Erwin were of his standard excellent quality. All boards were the same as those provided with the 3 hole version with the exception of the power supply filter board. This has an additional 2 filter caps and resistor for B+3a and 3b that provide power to the 4th tube.
Values for this amp are taken from #102 Robben Ford amp. Components are all current production. 20awg solid and 18awg stranded as appropriate.
FET is switched via back panel or footswitch with front panel level control.
Master volume is between Clean/OD and FX Bypass relays.
ODS Inspired Build; #102 Values with Buffered FX Loop
Moderators: pompeiisneaks, Colossal
ODS Inspired Build; #102 Values with Buffered FX Loop
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Glenn
Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Credits and references.
Thank you to Martin Manning and talbany and many others that provided information an drawings that made this possible.
Below are the drawings I used as references.
These values were used for the FX loop
Mostly I used the FX Bypass switch wiring scheme for the FX relay.
Thank you to Martin Manning and talbany and many others that provided information an drawings that made this possible.
Below are the drawings I used as references.
These values were used for the FX loop
Mostly I used the FX Bypass switch wiring scheme for the FX relay.
You do not have the required permissions to view the files attached to this post.
Glenn
Re: ODS Inspired Build; #102 Values with Buffered FX Loop
First i needed a preamp board with some additionl components. Using Erwins preamp board as a reference I marked a blank board, drilled holes and installed eyelets.
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Glenn
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martin manning
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Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Great stuff! What state is it in now? Any assembly work started?
Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Actually assembly is nearly complete. All that is left is 120vac input wiring and then ready to test. I already tested the 12vdc relay and manual switch installation with positive results. No pics yet but will take and post some this evening.
This was built "on the fly" without a master drawing, just worked with the information already posted piecing it together as I went. Its just how I work. I may make an after the fact as built schematic once it's done and tested, not sure yet.
This was built "on the fly" without a master drawing, just worked with the information already posted piecing it together as I went. Its just how I work. I may make an after the fact as built schematic once it's done and tested, not sure yet.
Glenn
Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Better late than never. Here it is prior to first power up.
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Glenn
Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Powered it up and tested it out.
Used my variac and formed the caps. New caps and may not have needed it but I wanted the experience. Amp came up smoothly with no issues.
Tubes were installed, 8 ohm dummy load plugged in and fired it up. Bulb on limiter reacted as expected and voltages were low, again as expected. Bulb limiter removed and amp powered up with full wall power. Voltages came up as expected.
Using sig gen and o'scope traced signals and all looked good except for an odd signal at higher input levels to PI. Turned out to be positive feedback. Swapped leads to output tubes and it cleared up.
Set PI balance.
Plugged in a speaker. Had some 60/120hz hum that tube rolling mostly corrected. FET has noticeable hiss that varies with level setting.
Testing out the controls revealed two different issues. All of the back panel switches were backwards and the FET relay was incorrect. The slide vs toggle switch issue got me-AGAIN!
While correcting the switches a careless positioning of the soldering iron damaged the .33 uF coupling cap to the FX Send pot. Tested it and now a short. Fortunately had a Solen Fast cap of the same value so it was put in.
Now all controls work as they should. Pedal isn't completed so not yet tested. Some of the switching does POP, namely the FET relay.
Clean channel is pretty quiet, with only a slight hum with Volume and Master dimed. Clean stays clean until volumes are set pretty high with plenty of Headroom.
FET sounds good, better than any other FET I've built to date. I'm sure it has to do with properly setting up the bias, this is the first one I did this to. Can be used to put some hair on the clean channel.
OD is good. From light to fairly heavy distortion is available. Adding FET brings it to another level!
FX loop bypass works great, can't tell it's there. Add the loop in adds a complete other level to alter the tone. CF stage drive level is controlled by Master volume, then there is a Send that, with nothing plugged into the loop, controls drive to return buffer stage. Receive level, with Bright switch, now controls the overall volume of the amp. I'm not sure how I feel about it yet due to lack of experience with it, so no opinion to post at this time.
I have not actually plugged any effects into the loop.
Building the amp was a real challenge. The fourth preamp tube make things really tight, making lead dress difficult. Finding pots for the FX Send and Receive levels took a few tries. I finally settled on a 250kb 16mm switched Receive and 50kb 16mm send. 24mm does not fit. I also added a mini toggle switch to control the FX Bypass relay as previously mentioned.
Used my variac and formed the caps. New caps and may not have needed it but I wanted the experience. Amp came up smoothly with no issues.
Tubes were installed, 8 ohm dummy load plugged in and fired it up. Bulb on limiter reacted as expected and voltages were low, again as expected. Bulb limiter removed and amp powered up with full wall power. Voltages came up as expected.
Using sig gen and o'scope traced signals and all looked good except for an odd signal at higher input levels to PI. Turned out to be positive feedback. Swapped leads to output tubes and it cleared up.
Set PI balance.
Plugged in a speaker. Had some 60/120hz hum that tube rolling mostly corrected. FET has noticeable hiss that varies with level setting.
Testing out the controls revealed two different issues. All of the back panel switches were backwards and the FET relay was incorrect. The slide vs toggle switch issue got me-AGAIN!
While correcting the switches a careless positioning of the soldering iron damaged the .33 uF coupling cap to the FX Send pot. Tested it and now a short. Fortunately had a Solen Fast cap of the same value so it was put in.
Now all controls work as they should. Pedal isn't completed so not yet tested. Some of the switching does POP, namely the FET relay.
Clean channel is pretty quiet, with only a slight hum with Volume and Master dimed. Clean stays clean until volumes are set pretty high with plenty of Headroom.
FET sounds good, better than any other FET I've built to date. I'm sure it has to do with properly setting up the bias, this is the first one I did this to. Can be used to put some hair on the clean channel.
OD is good. From light to fairly heavy distortion is available. Adding FET brings it to another level!
FX loop bypass works great, can't tell it's there. Add the loop in adds a complete other level to alter the tone. CF stage drive level is controlled by Master volume, then there is a Send that, with nothing plugged into the loop, controls drive to return buffer stage. Receive level, with Bright switch, now controls the overall volume of the amp. I'm not sure how I feel about it yet due to lack of experience with it, so no opinion to post at this time.
I have not actually plugged any effects into the loop.
Building the amp was a real challenge. The fourth preamp tube make things really tight, making lead dress difficult. Finding pots for the FX Send and Receive levels took a few tries. I finally settled on a 250kb 16mm switched Receive and 50kb 16mm send. 24mm does not fit. I also added a mini toggle switch to control the FX Bypass relay as previously mentioned.
You do not have the required permissions to view the files attached to this post.
Glenn
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martin manning
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Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Nicely done, Glen! A complex build like this powering up with just a couple of minor problems is quite an accomplishment.
OT phase test would have got the NFB right the first time ;^)
OT phase test would have got the NFB right the first time ;^)
Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Thanks Martin!
Odd thing is, this is the second build I've done with this model OT from Mojotone. First one I wired up by the colors and it was right the first time, so I expected this to be the same.
You are correct, the OT test would have gotten it right. Lesson learned. Had I done this test and thought out the relays/switches better this would have been a flawless startup. I haven't had one yet so I'll just have to keep trying
still saving my pennies for one of Erwin's SSS chassis...
I need to figure how to record some sound clips and get my son to play so y'all can hear these and give me some honest feedback.
Odd thing is, this is the second build I've done with this model OT from Mojotone. First one I wired up by the colors and it was right the first time, so I expected this to be the same.
You are correct, the OT test would have gotten it right. Lesson learned. Had I done this test and thought out the relays/switches better this would have been a flawless startup. I haven't had one yet so I'll just have to keep trying
still saving my pennies for one of Erwin's SSS chassis...I need to figure how to record some sound clips and get my son to play so y'all can hear these and give me some honest feedback.
Glenn
Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Ooh! 'Splain, pls.martin manning wrote: ↑Mon Mar 18, 2024 3:11 pmOT phase test would have got the NFB right the first time ;^)
I build and repair tube amps. http://amps.monkeymatic.com
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martin manning
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Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Like this:
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Re: ODS Inspired Build; #102 Values with Buffered FX Loop
My thinking is both voltages should be the same, any idea what the mechanism is that causes them to differ like that?
My band:- http://www.youtube.com/user/RedwingBand
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martin manning
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Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Same as an HT winding where the CT is grounded, the voltages at the opposite ends of the coil are out of phase. The one that is out of phase with the secondary shows a higher voltage because its sine wave is high when the one at the secondary hot is low.
I like this technique because it can be used with the OT in circuit.
I like this technique because it can be used with the OT in circuit.
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Raoul Duke
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Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Very interesting build Glenn! Impressive figuring it all out like you did. Way above my cranium, lol.
Re: ODS Inspired Build; #102 Values with Buffered FX Loop
Ahh, the voltmeters are with respect to the OT secondary hot, not 0V common.martin manning wrote: ↑Fri Apr 05, 2024 12:51 am Same as an HT winding where the CT is grounded, the voltages at the opposite ends of the coil are out of phase. The one that is out of phase with the secondary shows a higher voltage because its sine wave is high when the one at the secondary hot is low.
My band:- http://www.youtube.com/user/RedwingBand