Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on August 22, 2018, 11:10:06 am
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Has anyone built this preamp?
http://www.prowessamplifiers.com/schematics/images/Soldano%20SP77%20redrawn.pdf
I'm thinking if it is wise to replace the optocouplers and their control by using relays.
The relays can of course cause some popping which is difficult to know beforehand.
/Leevi
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> wise to replace the optocouplers and their control by using relays.
Why?
You can still get optos. The specs are not very critical here.
The optos naturally have a slow change which works well in audio.
To do relays you'd have to re-plan the whole thing and find the small additional power.
BTW: the connections shown around the HT side of the PT can NOT be right.
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> wise to replace the optocouplers and their control by using relays.Why?
I just want to get rid of the transistors and optocouplers due to some implementation related reasons.
I checked the logic of the switches and ended up to a solution that two relays (DPST) would be enough including even the led control.
The optos naturally have a slow change which works well in audio.
That is a good point! I have spent lot of hours trying to eliminate disturbances caused by the relays.
BTW: the connections shown around the HT side of the PT can NOT be right.
Yes that's true. Maybe it is better to check things from the original schematic as well.
https://el34world.com/charts/Schematics/files/Soldano/Soldano_sp77_preamp.pdf
What might be the secondary voltage of the PT?
/Leevi
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I found another mistake in the redrawn schematic.
Switch groups should be S2/S4 and S1/S3 not S1/S4 and S2/S3 like marked in the redrawn schematic.
/Leevi
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Some time ago I decided to build this preamp and recently I discovered this topic.
I will also use relays for channel switching, but I preffer to use 4 SPST relays instead of the optocouplers, and use the schematic with transistors to switch them.
Regarding the "redrawn" schematic, there is a mistake in the switching diagram. A bias resistor is missing.
About the power transformer, I ordered a custom toroidal transformer,and the tubes will be 6N2P-EV
The circuit will be built on turret board.
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Hello
Here are some pictures with the actual progress.
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What is the secondary voltage of the toroidal transformer?
/Leevi
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Hello Leevi
The high voltage secondary winding of the transformer is 250V /0.04A
I oversized the current current capability to avoid the heating of the transformer.
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I finished my built, but it doesn't work. :sad:
The clean channel is dead and the high gain channel has a lot of noise and squealing and low output.
Also, I connected the oscilloscope on the output and found some great static noise.
This issue could be because of the power transformer it wasn't properly designed? (anode power supply has less voltage than is needed)
I measured the plate and cathode voltages on each tube and I found some differences comparing with the voltages from preamp section of SLO ( Sp77 schematic is simmilar with the preamp schematic of the SLO).
I will post the schematic with the measured voltages.
If someone has an idea of what could be, or any tips, I remain grateful.
PS: Sorry for my english
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This is the schematic with the voltages mesured by me.
The plate resistor for V1a in my case is 220K
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i'm so lost reading this schematic, while building this, did you redraw the schematic so it would show more clearly how the the clean and lead channels are?
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Nevermind, i should start reading these topics from the first post :smiley:
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vladm, can you post pictures of your finished build, maybe someones fresh pair of eyes can spot a wiring error or something.
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yes, I will take some pictures with the finished build.
I made a Turret board layout from the "redrawn" version of sp77 schematic.
Also, if you want a clear view about the channels and the schematic of this preamp, you can check the schematic posted by Leevi (see #1 post)
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Ok, here are the photos that I took inside the build:
https://ibb.co/HKgdtm5
https://ibb.co/Nt0PCNT
https://ibb.co/wh9SXRT
https://ibb.co/9yjdq1R
https://ibb.co/dmR5s0S
https://ibb.co/kQ26kk0
https://ibb.co/j6VWkCr
https://ibb.co/xqsN76X
https://ibb.co/DDcRD4J
https://ibb.co/3f4H5b9
https://ibb.co/zGwRLsN
https://ibb.co/wy8DXbZ
https://ibb.co/VCCDTb4
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thanks, i take closer look at them when i have more time.
My approach to troubleshooting would be trying to get each channel working first without the relays. Hard wire for the clean channel first, once you get that working try to make the lead channel to work. Switching should be easier to get right after that.
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Ok. I will try this.
Thank you very much for the advice!
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Meanwhile, I made some progress.
I found a cold solder joint at the GND of the Master pots.Now the tonestacks and Master volumes are working, but the Clean channel sounds a little gainy and muffled. Also the Lead channel sounds a little muffled and squeals.
I connected the preamp output to the grid of the V3a (where Clean and Lead channels are meet,ignoring the V3 and V4 stages) and it sounds a little better (not so muffled, but the clean channel is still gainy). So, I suspect that there is something wrong with V3a, V3b and V4a (cathode followers) and V1b(clean channel stage). Dunno what to say about the input and output buffers (V1a and V4b).
Maybe the transformer doesn't have enough voltage to supply the plates (I read on SLOclone forum that the AC voltage of the secondary winding for SP77 has 304V AC, and in my case I measured 280V AC) and that's the cause for the "muffled" sound?
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Being a little low on B+ won't make "muffled". Just break-up a hair sooner.
DC voltage at all tube pins might be a clue.
But "muffled" is more likely a wrong resistor, capacitor, or connection.
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@PRR
I don't know if "muffled" is the correct word for this. It sounds like a dying battery fuzz.
Here are the DC voltages measured by me at the cathodes and plates tube pins.
For cathode resistors I used 0.6W resistors, except the 100K resistors from cathodes of V3b and V4a. They're 3W.
The plate resistor from V1b has been replaced with a 100K , and the plate voltage is 170V. The V3a plate resistor in my case is 100K ( instead of 47k) according to the "redrawn" version of the schematic.
Can you please tell me if there is something wrong with these voltage values?
Thank you!
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What model of transformer did you use? I've been.looking fir a small transformer for a new build. Thanks.
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Change R22 to 500 Ohms (tack another 1K across the one that is there).
Better or worse?
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@Captain chunkulus, the transformer used by me is a custom toroidal transfomer ordered from a local manufacturer from my country (Romania).
@PRR, thanks for the advice. I will try this modification.
Meanwhile I realized that it could be something wrong with the power supply because the AC voltage of the secondary winding is 280V, and the values of the supply voltages (A and B) are smaller than 280V*1.41.
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something wrong
280 is the spec, loaded 270, that's like 95% happycamper, I wouldn't fret 1.414
go back to soldering :laugh:
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The tubes used by me were 6n2P-EV.
I changed all the tubes with some brand new JJ ECC83s and the results are:
High Gain channel: no more squeals, noise , even the Gain pot is at maximum.the channel works as it should.
Clean channel: first it seems to be ok, but after I play some notes/chords, the volume is decreasing and becomes very fuzzy and muffled. If I switch on High Gain channel and then I switch back to Clean ch, it's coming back to normal, but after a few notes becomes muffled again.
I also measured the Cathode voltages of each cathode follower stage when the preamp its switched on clean channel and the cathode voltage before the sound becoming muffled is around 170V, and when its muffled I measured 214 V.
According the ECC83 datasheet, the maximum value for the Heater-Cathode voltage is 200V. This could be the cause for the muffled sound?
I connected the output jack of the preamp to each output stage of the clean channel and everything is ok until the output is connected to the cathode follower output of the clean channel.
Anyone knows what could be the cause of this decreasing volume issue?
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it's early, but I don't see any CF in schemo reply #19
214 ~~ probably ok, drop the Cathode R some to get it @200 if you're loosing sleep
your volume drop is probably loading
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@shooter
please take a look to the V3a, V3b and V4a stages from the schematic of #19 reply. Maybe I am wrong.
I know that the original schematic doesn't look too friendly, but you can look on the redrawn version here : http://www.prowessamplifiers.com/schematics/images/Soldano%20SP77%20redrawn.pdf
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I decreased the value of the Cathode resistor to 47k, and the value of R22 to 500 Ohms (as @PRR said) , but no results.
The problem persists.
I connected a voltmeter to the cathode resistor to monitor the cathode voltage. When the preamp is switched to clean channel, the voltage is below 200V (~176V). As much as I'm playing some notes, the cathode voltage is increased , and the sound level decreases until becomes muffled (reaches to ~205V).
It's obvious that the cause of the muffled sound is that the cathode voltage reaches over 200V.
I don't know why this happens only to the Clean Channel.
A heater elevation circuit will solve this problem?
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It's obvious
without a scope of the signal, nothings obvious, are you into OD, in a bad way?
once an amp's up and basically workin, "tuning" sound is best via scope, DC volts is just the place the signal rides, so if you rise DC, there's less "room" for the +half of the sine wave to go before smakin it's head on the ceiling
if you're happy with hum, leave the filaments alone.
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are you into OD, in a bad way?
Imagine a fuzz pedal with very low battery. you need to hit the strings hard to hear some notes. And this effect intensifies as much as the cathode voltage reaches over 200V.
I have oscilloscope and signal generator. I can take some shots with the signal.
Can you please tell me the optimum amplitude for the signal that I need to apply at the input of the preamp to take some proper conclusions?
Regarding the heater elevation circuit, I read that this method can also save the tube lives, considering that in my case the cathode voltage is very close to the maximum value of Cathode to Heater voltage (Vkf).
Thanks a lot for all your replies
Vlad
PS: I don't own a X100 oscilloscope probe to track the high voltage levels
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R29 is missing or not really connected.
Grid of X3a to ground should be near 2.2 Meg, not infinity.
Ah, I see. The schematic is wrong. When S2 is open, X3a has no grid-leak resistor. Put 2.2Meg directly from that grid to ground.
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R29 is connected.
You were right,PRR :bravo1: . X3a had no grid-leak resistor so I connected a 2M2 resistor from grid to ground. No more muffled sound or other weird stuff, but the channel is too gainy for a clean channel, so I had to reduce the value of X3a grid leak resistor. Compared with the clean channel of the originals SP77 that I saw on youtube, mine still has more gain.
@PRR,shooter,VMS: Guys, thank you very much for your support during the debugging process! :d3:
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:occasion14:
Ok it is required that you now post a sound clip!
Jim :icon_biggrin:
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Here is a quick sound clip with the high gain channel:
https://soundcloud.com/user-505539693/soldano-sp77-clone-high-gain-channel
Sorry for the potato quality of the sample. It was recorded by phone.
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Vladm , Leevi ,
Ive built a few of these 3+ triode hi gain amps / preamps.
I don't really like the "stock" coupling cap values that most of these Soldano / MesaBoogie preamps use. The end result, to ME, is a flabby low end. (i've owned a Mesa Rectifier too)
If your build would easily let you change the coupling caps, try 0.0068 uF coupling caps between the first and second stage, and between the second and third stage. There are some Bogner hi gain amps that use these smaller values, and the Carvin Ecstasy uses smaller coupling caps too.
The end result with the smaller caps is a more focused, rounded sound, with the low end tightened up.
But, of course, If you like the 0.02uF coupling caps and don't think the low end is a problem, that's cool too. :icon_biggrin:
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Vladm , Leevi ,
Ive built a few of these 3+ triode hi gain amps / preamps.
I don't really like the "stock" coupling cap values that most of these Soldano / MesaBoogie preamps use. The end result, to ME, is a flabby low end. (i've owned a Mesa Rectifier too)
If your build would easily let you change the coupling caps, try 0.0068 uF coupling caps between the first and second stage, and between the second and third stage. There are some Bogner hi gain amps that use these smaller values, and the Carvin Ecstasy uses smaller coupling caps too.
The end result with the smaller caps is a more focused, rounded sound, with the low end tightened up.
But, of course, If you like the 0.02uF coupling caps and don't think the low end is a problem, that's cool too. :icon_biggrin:
Hello nandrewjackson and thank you for your reply
Regarding the cap from the output bufffer (0.47uF), do you have a suggestion? for me, it seems to be too much.
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I don't have any schematics of the hi-gain preamps I've built, and I sure don't exactly remember the last cap in them,
Due to lack of $$ I've always just tore apart the preamp and re-used the power transformer for the next incarnation.
However, I do have a schematic for the last tube pedal I made, and while it's not hi-gain, it does have an anode follower output.
I used a 0.01uF cap on it, with 33K in parallel to ground with the output volume pot, to get a usable range on the output pot.
I hope this helps.
https://drive.google.com/open?id=1boUmG_k_4OSXDUnSrjr5d2OH_y7yZY5r
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Old thread but i'm looking to build this. Looking at this schematic. Can someone explain V3B ? It has no plate resistor (possible) but the grid is being fed by another plate resistor? (so is V4A grid). Is that correct?
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They are both DC coupled cathode followers. Many of the big Marshall amps use the same circuit to drive a tone stack. Go to the Valve Wizard's website for more info.
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Yup i see it. back to school. Just when you think you know. :l2:
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Note on the ”Clean” channel. By the schematic, it looks similar to the Crunch mode of the SLO-100. I.e. it’s not supposed to provide crystal clean tones, but rather mild breakup to perhaps ”AC/DC” type crunch sounds.