Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

High gain DHT submini preamp+amp with the 5678 and 5672
High gain DHT submini preamp+amp with the 5678 and 5672
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 18th December 2018, 09:29 PM   #1
Thomasha is offline Thomasha
diyAudio Member
 
Join Date: Jun 2012
Default High gain DHT submini preamp+amp with the 5678 and 5672

Hi,

after not so long, I'm starting my third attempt with these direct heated subminiature tubes. This time I'm going for a well known high gain amplifier from the mesa family. There are a ot of fet pedals around and here I try to achieve something similar with the subminis.

First test was use the original schematic and adjust the filament bias. I'm using some filaments in series, to save some current, and others alone with bias resistors. The tubes have a current of only 50mA and even five won't be much hassle for a 9v 1A supply. For the B+ I'm using a max SMPS and I decided to go with 45v, as the tube datasheet specifies.

So far I built the prototype and made a short video of the rig. No power stage so far, but it will probably be the 5672 running at 65v.

YouTube

I'm doing some simulations using spice to compare the real amp with my clone. There are some wrong values in the schematics I found, If someone has an accurate schematic of the dual rectifier let me know.

I'm simulating it at 415V, while my clone runs as 45v, so almost 10 times lower. The 5678 used in the preamp are running in triode mode, where the gain is really limited,but since my HT is almost 10x smaller, so is the gain. I would like to scale some filters down to avoid losing much signal. Measuring at 650Hz the 470k grid resistor of the second stage already reduces the signal to 1/3. At the gain potentiometer I measured 1.5v Vp-p, while at the second grid only 0.45vp-p.
  Reply With Quote
Old 20th December 2018, 12:15 PM   #2
Thomasha is offline Thomasha
diyAudio Member
 
Join Date: Jun 2012
Hi,

to clarify a little bit about this build, I started from this schematic:
[Ref:]
Click the image to open in full size.

where they discuss about the correct value for the gain potentiometer. It should be 250k. On the other hand, the schematic has voltages that could be used to verify how the stages are biased.

I tried to estimate the bias position on the load line and used it to calculate a similar bias for the 5678. Since the cathode follower is really difficult to implement with DHT I decided to make the last stage with less gain and more current, hoping that this would help when driving the tone stack.

Click the image to open in full size.

and the calculated results:
Click the image to open in full size.

To verify the frequency response of the preamp I used LTSpice. There was no model for the 5678, so I made a rough estimation of the curves for the triode mode.
Click the image to open in full size.

And here you find the final schematic:
Click the image to open in full size.

The pentodes are represented as a normal triode in this case, and the cathode resistor is only there to make sure that the bias is the similar to my build. In my build I used filament bias, meaning there are resistors between 20R and 165R dropping the PS 9V to the desired bias. It's different from a proper cathode bias, but it worked. I had even used some filaments in series to save some current and achieve the correct cathode voltage. Here all voltages are referenced to the negative side of the filament.

A sine wave with an amplitude of 150mA and 650Hz looks like this at the output:
Click the image to open in full size.

The measured signal is similar in shape:
Click the image to open in full size.

The voltage is a little higher, but the picture was taken before I added the filter before the last gain stage.

I still have some questions... if someone could please help me with them.

1) Is filament bias more like cathode bias, or grid bias?

2) Do I need to bypass the filament bias with a large capacitor? And where? Negative side, positive side or both? To ground or to the other filament side?

3) Is there a good substitute for a cathode follower, easy to implement with a DHT?

4) What do you think about using a lower gain stage, with higher current bias, before the tone stack? Should I expect significantly signal losses?

5) What about using the 5th stage to compensate for the signal losses in the tone stack?

6) Is this approach of scaling voltages and gain stages to emulate the distortion and signal response adequate? Since I come from a mechanical eng. background I would try to scale this problem, as the small models used in windtunels. Which are the electrical non-dimensional variables for such a problem?

I would like to thank you in advance for reading such a long post and trying to help me, even if it makes no sense at all to build such a thing.
  Reply With Quote
Old 10th March 2019, 11:51 AM   #3
Thomasha is offline Thomasha
diyAudio Member
 
Join Date: Jun 2012
Hi,
After some tests, the final result
Click the image to open in full size.
1 - I moved the tonestack one stage forward, so that there is still a driver stage for the 5672. The cathode follower has no gain anyway, and I didn't want to add another 5678 to the circuit, 6 tubes seems enough.

2 - I added a clean switch, to bypass some stages and get a nice clean or overdrive with the gain maxed.

3 - The SMPS I used was based on the 555, with some minor changes, my layout for the MAX1771 was too noisy.

4 - Some stages have a bias trimpot. The bias is calculated between the negative side of the filament and the grid voltage. Since the filament voltaeg is fixed by the series string, I adjust the grid voltage, as in a fixed bias amplifier. Third stage is biased at almost 2.25V, close to cut-off, a in the mesa. The 5672 is biased at the same time, and has a cold bias, as specified in the datasheet.

Here is the board
Click the image to open in full size.
Click the image to open in full size.

I fixed the metal pin of the socket directly to the board. A slightly undersized hole keeps it in place.

Here, another board with an extra RC filter for the 70V.
Click the image to open in full size.

I adjusted the trimpots before mounting it in the box. So I could also make some measurements.

The tonestack was monted in the box. Since I only had a PCB potentiometer for the mids I had to use a small board and solder it to the neighbours. I thought about doing a tonestack board, but there is no space for resistors or capacitors between the pots, so it makes no sence. This way I can still test different tonestacks
Click the image to open in full size.

I had to sand the lid a little bit, so that it would fit
Click the image to open in full size.

The etch was inspider by Homer and Scylla, the six headed monster. (six tubes ...)
Click the image to open in full size.
Click the image to open in full size.

The SMPS would not fit in the box, so I butchered it!
Click the image to open in full size.
Click the image to open in full size.
I had to cut it, and turn it 90 degrees, so that it would still fit.
Click the image to open in full size.
The tubes were soldered to the sockets to make things easier
Click the image to open in full size.

And the final result
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
  Reply With Quote
Old 11th March 2019, 06:36 PM   #4
djgibson51 is offline djgibson51  New Zealand
diyAudio Member
 
Join Date: May 2010
Location: Hawkes Bay
Wow. Thanks for sharing. I have several Oately Electronics sub-mini kits waiting for a rainy day.
  Reply With Quote
Old 16th March 2019, 09:02 PM   #5
Thomasha is offline Thomasha
diyAudio Member
 
Join Date: Jun 2012
Here a small video of how it sounds
YouTube
  Reply With Quote
Old 17th March 2019, 08:09 PM   #6
sahazel is offline sahazel
diyAudio Member
 
Join Date: Mar 2019
What a cool project! It came out looking and sounding great. The concept of doing lower-power classic amp designs is really interesting. Might make for some nice alternatives to practice amps like the Blackstar Fly and Boss Katana Mini.

I've been exploring subminiature tubes for guitar amps as a way to experiment with tubes without working with very high voltages just yet (since I'm new to analog electronics). I started with the Schiit coaster headphone amp project and have added extra 6088 preamp stages and paralleled tubes to get the power output up. I'm looking at replacing the transistor push-pull output stage with an SET and had identified the 5672 as a likely candidate, which is what led me here. May I ask what you're using for the transformer?
  Reply With Quote
Old 18th March 2019, 06:10 PM   #7
Thomasha is offline Thomasha
diyAudio Member
 
Join Date: Jun 2012
The output transformer is a 022921 reverb transformer, from fender I guess. There is an hammond equivalent though. It has basically 22k primary to a 8 ohms secondary.

You could probably use a line transformer too, it just needs to have the low wattage pins (1.25w, 0.65w) that correspond to higher impedances.
  Reply With Quote
Old 24th March 2019, 09:57 AM   #8
sahazel is offline sahazel
diyAudio Member
 
Join Date: Mar 2019
Got things working with a 25k:8ohm transformer! I've currently got two 6088 preamp stages and an output stage with parallel 5672s, which is producing enough clean volume to serve as a good starting point for experimentation.

Thanks for posting all this info. I'm figuring things out pretty quickly now thanks to clues from your schematics. Seeing how you're biasing relative to filament voltage was particularly helpful, since I was struggling to understand how to bias these direct-heated subminiatures. Took me a while to make sense of the negative grid bias, but fortunately I decided to try imitating your series filaments. Once I got my filament negative up above 0v and my output lifted further from the negative rail, I had the hint I needed.
  Reply With Quote
Old 24th March 2019, 06:24 PM   #9
sahazel is offline sahazel
diyAudio Member
 
Join Date: Mar 2019
I'm curious what kind of precautions you take when working with your power supply. Do you treat it like the high voltage rails / filter caps in most tube amps?

I've got my positive rail at 30v (produced with rectification and a linear voltage regulator from a 16vac supply). I'm borderline starving my tubes, so I've been thinking about bringing the voltage up to the 45v or 65v the data sheets specify, but I want to be careful as I start to ramp up the risk of shocks and damaging components. I shorted some component leads on my breadboard on accident and caused a little spark, and I wonder what that would have been like at 65v...
  Reply With Quote
Old 24th March 2019, 10:20 PM   #10
Thomasha is offline Thomasha
diyAudio Member
 
Join Date: Jun 2012
Always treat it like it's going to kill you. It can get dangerous if there is current available.

Tubes are kind of resistant to those shocks, but the filaments could be damaged.
Since I'm using SMPS if I short something the SMPS dies first. I burned a reasonable amount of ICL7660s and MAX1771 chips just by shorting something, or a loose cap discharging.

When I breadboarded it with a transformer the hum was so loud I had to add 1000uF caps to get it tamed, so I got back to the smps...but if using a transformer it could get more dangerous.
  Reply With Quote

Reply


High gain DHT submini preamp+amp with the 5678 and 5672Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
op-amp based, high-gain, high rail voltage, preamp CharlieLaub Analog Line Level 60 12th December 2017 09:09 AM
High Gain Guitar Preamp Stormrider Instruments and Amps 69 19th June 2012 03:15 PM
High gain guitar preamp kits! Hybrid_Child Instruments and Amps 1 15th July 2009 05:18 PM
submini tube preamp is a success unclejed613 Tubes / Valves 4 26th April 2007 03:56 PM
High Gain Preamp amirmk Tubes / Valves 3 27th August 2005 12:00 AM


New To Site? Need Help?

All times are GMT. The time now is 10:49 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki