UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 4th Mar 2016, 4:32 pm   #1
vampyretim
Heptode
 
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
Default Solid state preamp for EL84

Hi guys,

I've built some solid state preamps using BC109's. I've taken my supply voltage from the cathode of the EL84 and switched the 25uF for 220uF. The amps always sound unstable. I have put a .001uf cap across the output of the BC109 but to no avail.

I have even tried hooking up a 9V battery to the solid state preamp again horrible distorted sound.

Has anyone ever successfully designed a solid state preamp for use in single stage valve amped record players?

Can anyone share that design with me?
vampyretim is offline  
Old 4th Mar 2016, 4:41 pm   #2
Guest
Guest
 
Posts: n/a
Default Re: Solid state preamp for el84

Can we have the circuit diagram please? With the valve included.
 
Old 4th Mar 2016, 4:52 pm   #3
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Re: Solid state preamp for el84

Agreed.
Unless we know the exact application and circuit its impossible to diagnose a problem.
It does sound as though you are overloading the valve amp though. Also my view of "stealing" power from a cathode tap is that its not ideal. if you cant use a battery, then there's lots of titchy little PCB mount (veroboard ok) mains transformers to give you up to 10mA of current. Use with a cheap voltage reg and a FW rectifier.
There's no practical reason at all not to use a solid state preamp in front of a valve power amp if you need a bit of gain, or maybe an impedance buffer.
My everyday Preamp in the Hi Fi is a Rega Cursa, ideal for me as it has user configurable gain settings, so that it is as good in front of a leak power amp, up to something like quads needing a larger input.
Andy
bikerhifinut is offline  
Old 4th Mar 2016, 5:00 pm   #4
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Re: Solid state preamp for el84

Quote:
Originally Posted by vampyretim View Post
Has anyone ever successfully designed a solid state preamp for use in single stage valve amped record players?
Sounds like you need to bump up the output of a ceramic med to low output cartridge to that of a high output one.
I guess a total gain of between 2x and 4x would be ample? Can the cartridge experts verify that please?
This has been covered before, and I'll say the same again. I'd make an amp using a cheap readily available opamp say a TL071 or similar, heck for a record player I bet a 741 would work well.
Then set the gain with the feedback resistors to what you need. Also you can configure the input resistance to suit a high impedance ceramic cartridge.
You could do all this with one of the cheapo preamp kits from the much maligned Maplin organisation and simply substitute the required resistor values setting gain and input resistance.
Its maybe a couple of quid more than a BC109 and a few resistors but you will get consistent results.
Andy.
bikerhifinut is offline  
Old 4th Mar 2016, 5:07 pm   #5
Hartley118
Nonode
 
Hartley118's Avatar
 
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,196
Default Re: Solid state preamp for el84

No problem drawing a few mA of supply power from the EL84 cathode as long as it's well decoupled (probably with an extra stage of RC after your 220uF). But you're inevitably on to a loser driving the EL84 grid with an amplifier powered from the EL84 bias supply itself. The transistor output stage will need a power supply voltage comfortably higher than the grid swing needed by the valve, and the bias voltage won't be high enough for that.

Martin
__________________
BVWS Member
Hartley118 is online now  
Old 4th Mar 2016, 5:23 pm   #6
Guest
Guest
 
Posts: n/a
Default Re: Solid state preamp for el84

Quote:
The transistor output stage will need a power supply voltage comfortably higher than the grid swing needed by the valve
You could double the cathode resistor and bias the feeding amplifier to half way, coupling directly to the grid.
 
Old 4th Mar 2016, 6:14 pm   #7
Edward Huggins
Dekatron
 
Join Date: Sep 2015
Location: Southwold, Suffolk, UK.
Posts: 8,302
Default Re: Solid state preamp for el84

Three cheers for this thread!

I think over the next few months a solution to the problem of no more availability of high-output cartridges in the UK is going to become really acute. Given the so-called vinyl revival, just think of all those single-stage UL84/EL84 Dansettes, Fidelitys, Albas et al out there just waiting for a solution. Indeed, there's a really big market here for an easy-to-install one-stop, "back box" product.
My first thought about this was for a simple opamp giving x2 or x4 stage gain and running off 2 x AA cells. This would mean no interference to the cathode arrangements on the existing installed amplifier. Then I thought about using the valve's Cathode voltage supply to get around the faff of battery changing and the need for a separate on/off switch. However, from the comments in this thread, I can now see this may not be such an elegant solution as I had first thought.
Customers come to me with complaints about low volume coming from so-called "restored" record players. Yes, they've fitted with the wrong cartridge.
I think this will soon become a burgeoning issue within this Forum. Nearly a third of recent posts in this AUDIO section have been about crystal/ceramic cartridges and the issues arising. Edward
Edward Huggins is offline  
Old 4th Mar 2016, 8:21 pm   #8
vampyretim
Heptode
 
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
Default Re: Solid state preamp for el84

Hello guys,

Thanks for the responses. It's a Dansette Bermuda. It has two resistors 3.3k for g2 and 120 ohm for cathode bias. It's the simplest of amps! Am tearing my hair out. I thought it might be an impedance mismatch but it sounds too extreme for that.

Sorry I should say I've put 470ohm in the cathode to get the anode voltage up to 250V.

Any ideas?

Sorry guys, will post a full circuit including the BC109 tomorrow.

Last edited by vampyretim; 4th Mar 2016 at 8:36 pm.
vampyretim is offline  
Old 4th Mar 2016, 8:38 pm   #9
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default Re: Solid state preamp for el84

What about getting your supply from a half wave voltage doubler on the heater supply?

That'll give around 15V at 10mA or so using a couple of 1N4000 series diodes and a couple of 470uF capacitors.

That should be enough to get about 12V p-p output to the valve from the right circuit.

Another one backwards and you could run an opamp from 15-0-15 rails and directly couple to the grid with a possible 24V swing!
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is online now  
Old 4th Mar 2016, 9:57 pm   #10
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,171
Default Re: Solid state preamp for EL84

Hi Tim, I have one I built many years ago using the Philips TAA320 fet/ bipolar "transistor".
This drew its few mA from the HT rail and worked very well.

these chips are still available but officially obsolete now.

Ed
Ed_Dinning is offline  
Old 4th Mar 2016, 10:03 pm   #11
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
Default Re: Solid state preamp for EL84

Couldn't you use a FET? Something like a 2N3819 is spiritually close to the triode you'd find in an ECL82 or similar-era triode-pentode valve that would typically be used in a budget record-player with a 'low/medium output' cartridge.

It gives you gain and a nice high input-impedance which can then be loaded with a resistor (or a R-C network to give RIAA-style rolloff) to match the cartridge's impedance.

A high-value resistor (and maybe an 18V zener too) from the EL/UL84's HT supply will power it: you only need a few milliamps.

Last edited by G6Tanuki; 4th Mar 2016 at 10:15 pm.
G6Tanuki is offline  
Old 4th Mar 2016, 10:05 pm   #12
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Re: Solid state preamp for el84

Quote:
Originally Posted by Herald1360 View Post
What about getting your supply from a half wave voltage doubler on the heater supply?
Interesting ideas Chris.

Devils advocate here.
In the interest of simplicity and minimal interference with the original circuit, and also with the feeling that there may be people wanting to do this who may not have the appreciation of the potential effects on their personal safety and on the operation of the amps, I would suggest (again) this:
Obtain a cheap subminiature 9V to 15v transformer, you only need 10ma or so probably less so the smallest you can get is good.
Bridge rectify it (Full wave), regulate with a cheap 3 legged fixed voltage regulator so no faff with setting resistors for output voltage. A couple of caps for reservoir/smoothing. Voila! PSU.
Then a simple opamp circuit set to the required gain and input resistance.
this could all fit on something not a lot bigger than a matchbox, given the size of some of the potted subminiature mains transformers available.
Build it on a bit of vero or perforated board, or be posh and make a small PCB.
There should be room inside most vintage record players to mount it, hopefully near the arm wires.
I'm rubbish at drawing and making PCBs but I bet theres a fair few in the forum could devise a board design that just about anyone with soldering skills could build up.
Andy.
bikerhifinut is offline  
Old 5th Mar 2016, 2:20 pm   #13
vampyretim
Heptode
 
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
Default Re: Solid state preamp for EL84

It would be desirable for me to take the solid state power from the existing amp . So long as it didn't interfere with the valve. It would also be desirable to have as few components as possible for it to be small and cheap.
I love all of the ideas I've read on here. Can you all start posting schematics.
vampyretim is offline  
Old 5th Mar 2016, 4:15 pm   #14
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
Default Re: Solid state preamp for EL84

Some more ideas on this post.
https://www.vintage-radio.net/forum/...d.php?t=111572
Nuvistor is online now  
Old 5th Mar 2016, 4:16 pm   #15
Guest
Guest
 
Posts: n/a
Default Re: Solid state preamp for EL84

Post yours, it's easier to modify than start again.
 
Old 5th Mar 2016, 4:48 pm   #16
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,784
Default Re: Solid state preamp for EL84

There is absolutely no problem with stealing power from the valve cathode to power the preamp.

As others have said, we need to see your circuit. A simple one transistor preamp shouldn't become unstable, but you can overload it or have a serious impedence mismatch.

An FET or op-amp design is technically preferable but a simple bipolar transistor should be OK in a record player.

See this thread: https://www.vintage-radio.net/forum/...ad.php?t=45841
paulsherwin is online now  
Old 5th Mar 2016, 6:14 pm   #17
G4XWDJim
Octode
 
G4XWDJim's Avatar
 
Join Date: Oct 2008
Location: Littlehampton, West Sussex, UK.
Posts: 1,465
Default Re: Solid state preamp for EL84

One of the Seeburg jukeboxes used a pair of transistors in the front end. It worked well. I'll have a search in my books for the circuit it used.

For the jukebox aficionados it was the model that had a pair of stereo speakers sticking out like ears.

Jim
G4XWDJim is online now  
Old 5th Mar 2016, 8:57 pm   #18
vampyretim
Heptode
 
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 538
Default Re: Solid state preamp for EL84

Hi guys,

I couldn't find my original design at the workshop today so will have to copy it from the circuit that I have built.

I will do it when I'm back in the workshop on Monday.

Thanks ever so much for taking the time to help me.
vampyretim is offline  
Old 6th Mar 2016, 3:00 am   #19
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default Re: Solid state preamp for EL84

Quote:
Originally Posted by paulsherwin View Post
There is absolutely no problem with stealing power from the valve cathode to power the preamp.
Not normally for a multistage amp, but in this case with only the EL84, a simple class A preamp fed from the cathode volts is, as has been mentioned, physically incapable of delivering enough output swing to the grid of the same valve to deliver full output which requires a p-p grid input of nearly twice the cathode voltage.

In this case, simplest is still a single transistor, but it needs to be fed from the HT at a voltage of somewhat higher than twice the cathode voltage.

Something like an MPSA42 could use HT directly and with 150K collector resistor and unbypassed 15K emitter resistor and bootstrapped biassing via 10Meg + 47K resistor from collector to base will give a gain of about 10 with an input impedance of about 1Meg.
Attached Thumbnails
Click image for larger version

Name:	Bootstrap preamp.jpeg
Views:	504
Size:	51.0 KB
ID:	121137  
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is online now  
Old 6th Mar 2016, 9:59 am   #20
G4XWDJim
Octode
 
G4XWDJim's Avatar
 
Join Date: Oct 2008
Location: Littlehampton, West Sussex, UK.
Posts: 1,465
Default Re: Solid state preamp for EL84

This is the circuit as used in the SHF45 Seeburg DS 160 jukebox amplifier. It uses a 2N591 transistor in each channel fed by a magnetic pickup with 5 mV output and would be easy to copy.

The transistor supply is from the 30 volt line. I have included the relevant section of the parts lists in case anyone wishes to copy it.

The diodery CR103 etc at the bottom of the diagram is part of the AVC circuit.

It was about 30 years ago that I restored this machine and passed it on but I do remember it having a very good and 'proper' jukebox sound.

Hope it helps.

Jim
Attached Thumbnails
Click image for larger version

Name:	se3.jpg
Views:	550
Size:	51.0 KB
ID:	121144   Click image for larger version

Name:	se2.jpg
Views:	201
Size:	78.1 KB
ID:	121145   Click image for larger version

Name:	se1.jpg
Views:	178
Size:	57.9 KB
ID:	121146  
G4XWDJim is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 1:19 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.