Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: newguitarsmell on December 01, 2018, 08:28:46 pm
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I am curious about some of the differences between the Hoffman 5e3 schematic and the Fender schematic.
If I have started this post before, sorry. I can't find it.
Question 1: The grid stopper resistors coming from V1 (12ay7) are changed from 68k to 33k. I am new to this and I am curious why? Some schematics show a 68K to each input, but the Hoffman 5e3 has just a single 33k?
Question 2: Why was the cathode resistor for V3 changed from 250/5W to 250/10W?
Question 3:Compared to the Fender diagram, the + and - sides of the 16uf/450 electrolytic resistors are opposite.
Question 4: I understand from RobRobinette's site that a "lead" channel can be created by changing the .1 coupling cap on pin 1 of V1 to something lower. A resistor is also supposed to be changed -- has anyone done this with the Hoffman 5e3?
If these are dumb questions, sorry. I have read a lot of stuff but we all know there is book knowing and real knowing. I'm working on the real knowing part. I need an amp building mentor - and you guys are it. I truly appreciate the help. I am fascinated by all of this. The first two 5f2a amps I built are fantastic. It's so great when you can take more control of your guitar tone in that way. :worthy1: :worthy1: :worthy1:
Thank you! Margaret
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Then dust off the slide rule, I was "lead" here, I still go back ~bi-weekly
http://www.valvewizard.co.uk/gainstage.html
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Question 1: The grid stopper resistors coming from V1 (12ay7) are changed from 68k to 33k. I am new to this and I am curious why? Some schematics show a 68K to each input, but the Hoffman 5e3 has just a single 33k?
Because Doug is only using a single jack on each channel. So the input grid stopper R is 1/2 the value, 34K, but 33K is a standard value, close enough, you'll never hear any difference. The input signal is sooooooo small there. That R is a grid stopper, it is there to stop any possible oscillation. It is best at doing this tight to the grid 1/control grid pin, directly soldered to that socket pin. Fender didn't do that though. Do a search on grid stoppers. I'm sure it's in Merlins on line info.
Here's a link to Sluckey's amp scrap book. A lot of very good, clear information with very good, clear drawings. Scroll down to "Understanding Hi-Low Jack Switching". It will explain it to you in detail.
http://sluckeyamps.com/misc/Amp_Scrapbook.pdf (http://sluckeyamps.com/misc/Amp_Scrapbook.pdf)
Question 2: Why was the cathode resistor for V3 changed from 250/5W to 250/10W?
No change in sound/tone. Ohm value remains the same. It's for better reliability, increased from 5 watts to 10 watts dissipation, heat. Fender used 5w to keep the price down. Fender was making a LOT of amps, so it added up for Fender in production savings. And because they never thought anyone would ever turn that small amp up past 5 on the volume, so they didn't need a 10w R there anyway.
Question 3:Compared to the Fender diagram, the + and - sides of the 16uf/450 electrolytic resistors are opposite.
No change in the amps sound, Doug just flip/flopped the B+ to the other side of the eyelet/turret board. Buiders/designers choice. I would guess he likes the layout and wiring that way better.
Question 4: I understand from RobRobinette's site that a "lead" channel can be created by changing the .1 coupling cap on pin 1 of V1 to something lower. A resistor is also supposed to be changed -- has anyone done this with the Hoffman 5e3?
I recommend this before you try changing any coupling caps and R's on 5E3 1st gain stage input. Besides, 5E3's have plenty of gain/distortion. Those lead channel modes are more for a BF Fender, their cleaner.
A 5E3's 2 volume controls are interactive with each other. When you plug into either channel, the volume control on the channel your NOT plugged into will effect the gain on the channel you ARE plugged into. Try plugging into 1 channel, set that channel's volume for 12:00 and the other channels volume at max, it will be clean and scoop the mids. Leave the volume controls as their set but plug into the other channel, max distortion and max mids from that amp. Then try the same thing while plugged into the 2nd channel. It will be similar, but a little different because of the way the volume control is wired. And you can dial either volume control up or down a little and see what you like best.
You can then use a simple A/B box as a channel switcher with the interactive volume controls.
Study the schematic and see if you can figure out how/why the volume are interactive. Hint, the guitar signal has a 2nd path to ground. :icon_biggrin:
https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5e3_schem.pdf
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This is awesome! Just what I wanted to know.
THANK YOU WILLABE!
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And SHOOTER - -- great book! wonderful stuff.
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> If I have started this post before, sorry. I can't find it.
FYI:
Top of forum (while logged-in), find "Profile". In the Profile Summary, find "Show Posts". This is all your posts.
When you grow to much more than 14 posts, this becomes overwhelming. You can also try Search, 'some search words', By-User 'newguitarsmell'.