Record player tube amp modded for iPod use

[gutsofgold]

I have an old Voice of Music amplifier - 6X4 rectifier, 6AU6 pre, 6AQ5 power. It's really basic but it sounds pretty good, my only complaint is that it clearly was designed to be used with an old turntable cartridge as the source. I would like to run my iPod into it so I need to change the input of the amp to better match the line level audio coming from my iPod and I would like to also remove any EQ'ing that might be taking place in the circuit.

http://img821.imageshack.us/img821/223/d6h.png

Could some of you tube guys help me figure out the best way to get this amp suitable for line-level audio and make it flatter ?

[tubegeek]

No EQ in that circuit: the ceramic pickup used has a pretty high output and is close enough to self-equalized for cheap record player use. So that's what they did.

You can try soldering your 1/4" TS guitar input jack with the hot (tip) at the "input" 2M pot, lug 3 and the shield (sleeve) at lug 1 - or any convenient ground - for testing. Easy peasy. Should be a pretty sweet little guitar amp as is, in other words.

LUCKY!

[PRR]

This can be VERY dangerous!!

You need to know if the old record-player is "Hot Chassis". If it is, then any audio connection can be LETHAL.

Thanks for the complete schematic. This VoM is *not* hot-chassis. You can still stick your finger inside and die, but external connections *should* be "safe".

> soldering your 1/4" TS guitar input jack

I don't see guitar mentioned, but "iPod" mentioned several times.

iPod is stereo. You must mix Left and Right together. Shorting them will distort any stereo sound. Use two resistors. 10K, or 2K, or maybe 50K, whatever you have a spare pair of. There's a small loss but I think you have a ton of spare gain here.

There is a bit of "EQ" in the pot network from last plate to first cathode. Mostly it clobbers treble less or more, which is what you want on 78s and 45s. Also the stock speaker may have been bright. If you find the treble too weak, change the ".0035u" cap. 0.001uFd will shift the action high in the audio band for a mild treble trim. OTOH, 0.1uFd will shift the action to the *whole* audio band; now it is a "gain" control which you should probably set and forget. Turn Volume to about "9", play iPod at normal setting, adjust "gain" for LOUD, and mark/lock it. This also applies NFB to the whole audio band which may (or may not) smooth and damp your speaker response.

http://i.imgur.com/OE5EVLi.gif

[GibsonGM]

+1   All this old stuff is awesome, but they often didn't use transformers.  If one comes across one that is HOT CHASSIS, you must use an isolation transformer and get the polarities right, or risk the same effect as plugging your guitar into a wall outlet!!!!   

I bet that amp will sound AWESOME for you after you tweak it a little, and make sure it's safe )    Just reiterating what PRR is saying...awareness is JOB 1 if you get into this area of modding/building...

[amptramp]

The amplifier schematic looks good.  C5 and C6 could be higher value to reduce hum but do not increase C4 - it is pretty much near the limit for a 6X4.  A fuse or circuit breaker might be a good addition at the power input.  A noise filter on the input power leads that does not have a ground connection may be a good addition and film capacitors across the electrolytics would be a good idea to help eliminate high-frequency noise conducted along the power lines.

[R.G.]

The advice so far is good.

I just wanted to add that my favorite amp build ever was a conversion of a hifi amp from a Magnavox hifi console dating from the late 50s or early 60s. This thing was audio furniture, with honey-maple Colonial style and a 6AQ5 output stage.

The 6AQ5 is a baby brother to the 6BQ5/EL84/7594. It's good for a few watts - which is 95db or so with most 12" guitar speakers. With enough preamp gain, it easily gives that amp-melting-down sound of early blues recordings.

I eventually combined the output stage with my favorite preamp build ever, two 12AX7s in cascade, but with fairly drastic attenuation per stage to keep the distortion products but prevent oscillation and massive noise build up.

I really hate ever letting that one go.

[Mark Hammer]

The issue of Vintage Guitar, that I got in the mail on Tuesday, has a feature article on an amp that Radio Shack sold in the early to mid 60's, that was a dual EL84 guitar amp, but had a fold-down turntable!    Dave Hunter describes the "Realistic Entertainer 34" as being a bit like a mullet haircut "all business in the front and party in the back".

[wavley]

The amplifier schematic looks good.  C5 and C6 could be higher value to reduce hum but do not increase C4 - it is pretty much near the limit for a 6X4.  A fuse or circuit breaker might be a good addition at the power input.  A noise filter on the input power leads that does not have a ground connection may be a good addition and film capacitors across the electrolytics would be a good idea to help eliminate high-frequency noise conducted along the power lines.

I agree with this except I would ditch the power cable all together and go with a properly grounded mains cable, it's really the safest thing to do.  I personally would really only bother with upping the value of C6 a bit.  I would worry more about just replacing C4 and C5 with modern caps about same value.  With fresh modern caps around the same value I think this circuit would be fairly hum free.  Bypassing the electrolytics is a pretty good practice unless you go with low esl caps.  Or you could just ditch the 6x4 and replace it with a bridge rectifier and go crazy with filtering.  Of course I think this is a bit diminishing returns, a summed to mono iPod through this thing will probably sound good, but not exactly HiFI.  If you decide to use it as a guitar amp, the looser filtering and sag I think might be more fun for this circuit, of course you may like it to be tighter so take my opinion with a grain of salt.

I know everybody has said to be safe, I didn't skim over that.  I'm repeating it because it's actually that important.  Be safe, read this first. http://www.aikenamps.com/SafetyTips.html

[wavley]

The issue of Vintage Guitar, that I got in the mail on Tuesday, has a feature article on an amp that Radio Shack sold in the early to mid 60's, that was a dual EL84 guitar amp, but had a fold-down turntable!    Dave Hunter describes the "Realistic Entertainer 34" as being a bit like a mullet haircut "all business in the front and party in the back".

Because direct coupling of a turntable to a speaker cabinet was a great idea!

[Mark Hammer]

The issue of Vintage Guitar, that I got in the mail on Tuesday, has a feature article on an amp that Radio Shack sold in the early to mid 60's, that was a dual EL84 guitar amp, but had a fold-down turntable!    Dave Hunter describes the "Realistic Entertainer 34" as being a bit like a mullet haircut "all business in the front and party in the back".

Because direct coupling of a turntable to a speaker cabinet was a great idea!

Absolutely.  Every bit as good as the fold-down dashboard turntable for cars ( http://gajitz.com/road-tunes-weird-vintage-1950s-in-car-record-players/ ), and automotive spring reverb.

[gutsofgold]

You guys are awesome, thanks. Just to state the obvious - this thing does have a power transformer and I will be adding mains cable with proper ground.

I just replaced all electrolytics and any film caps, I left the two ceramics they seemed fine. Could I replace the volume pot with say a 1M to ground and place a volume pot before the 6AQ5? This way the ipod will constantly see the same input impedance?

[tubegeek]

gutsofgold -

Sorry about overlooking the iPod input - PRR is 100% on the money with his suggestions - gotta do a blend to mono with 2 resistors. See below.

You could do what you suggested, make the input resistor a fixed value, or it could be useful to be able to trim the input for the iPod level. Adjustable NFB is another way to control gain through the whole circuit.

Also sorry about overlooking what the 2nd pot was doing - again, PRR is right and I'm wrong. If you don't want the treble control that it provides, you could always reuse that pot and move it into the grid circuit of the 6AQ5 and disconnect it from the plate.

In detail: you would disconnect R6 and R8. Re-use R8 at the input jack. (See below.) Connect Lug 1 of the 1M pot to ground. Connect Lug 2 of the 1M pot to pin 5 of the 6AQ5. Open the connection of C2 to pin 5 and move the cap's lead to lug 3 of the 1M pot.

Note that  the mods above will remove the NFB on the 6AQ5. So re-use the 2M pot as a variable resistor between pin 3 of the 6AU6 and the left-hand end of C3, dial in the NFB you like with that pot. Maybe raise/lower the value of C3 per PRR's suggestions if you like.

Also:

Match 2 resistors of between 2K and 20K. Connect one end of one resistor to the Tip of your female iPod TRS input connector. Connect one end of the other resistor to the Ring of the connector. Connect the free ends together and connect this to one end of the 1M resistor you freed up from R8. The other end of the 1M resistor goes to ground.

Watch out for lethal voltages, especially at the NFB connection at the plate of the 6AQ5 but also everywhere else!

[PRR]

> Could I replace the volume pot with say a 1M to ground and place a volume pot before the 6AQ5? This way the ipod will constantly see the same input impedance?

iPod can drive 32 ohms or higher. 2Meg is much-much-much higher than 32 ohms.

2Meg shunted with a naked grid is the *same* impedance at any setting.

I have a 3-ton truck. Does it care if it carries a 6-pack? A 6-pack plus a bag of chips?

If the volume pot doesn't scratch, leave it alone.

I feel the same about the filter caps, except they are more likely pooped. Replace with similar values.

Lily-gilding--- been there done that and can't be excited. Especially since $20 will buy a Class-Q micro-amp of ten times the power and a hundred times cleaner. If you feel you *must*, you don't need encouragement; otherwise I'd love it as-is (with repairs).

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FWIW: 6AQ5 and 6BQ5 are not from the same father. AQ is a mini 6V6, good American tube. BQ is a re-numbered EL84, a very European tube. Yes, the similar letters may have been to "seem familiar", just as 6V6 echoes 6L6 (but is not really a small 6L6).

In perticular: 6BQ5/EL85 has much higher gain and much higher G2 current. Also 50% more power throughput if you wish.